1 00:00:09,000 --> 00:00:12,760 Seffner, Florida. 2 00:00:12,760 --> 00:00:15,200 A place where the earth opened up... 3 00:00:16,640 --> 00:00:19,360 ..and killed a man. 4 00:00:19,360 --> 00:00:21,600 Lazy Lanes, this place is called. 5 00:00:21,600 --> 00:00:24,080 It just seems so ordinary, so normal. 6 00:00:24,080 --> 00:00:25,240 DOG BARKS 7 00:00:25,240 --> 00:00:28,680 I mean, look at this, "Beware of the dog." 8 00:00:28,680 --> 00:00:35,400 And yet this whole estate sits above a trap door into the hidden Florida. 9 00:00:35,400 --> 00:00:37,880 'The bedroom floor just collapsed 10 00:00:37,880 --> 00:00:40,920 'and my brother-in-law is underneath the house.' 11 00:00:40,920 --> 00:00:44,360 The trap door into that hidden Florida opened here, 12 00:00:44,360 --> 00:00:47,000 in February 2013... 13 00:00:48,920 --> 00:00:52,480 ..creating a sinkhole. 14 00:00:52,480 --> 00:00:56,600 Without warning, it swallowed everything in this bedroom, 15 00:00:56,600 --> 00:01:01,600 and for those who saw it, it was something they'll never forget. 16 00:01:01,600 --> 00:01:04,240 It's like this thing was alive. 17 00:01:04,240 --> 00:01:07,280 You know, it was turning, moving around, 18 00:01:07,280 --> 00:01:09,920 making noises, you know, almost like a growl. 19 00:01:15,920 --> 00:01:18,200 Sinkholes don't just happen in Florida. 20 00:01:19,720 --> 00:01:21,880 They're occurring all over the world. 21 00:01:25,400 --> 00:01:27,920 I want to find out why sinkholes form... 22 00:01:30,680 --> 00:01:33,640 ..what this underworld is really like. 23 00:01:33,640 --> 00:01:34,960 You've not been down there? 24 00:01:34,960 --> 00:01:37,360 Never been in there, didn't even know it existed. 25 00:01:37,360 --> 00:01:41,040 And why some are deadly. 26 00:02:06,840 --> 00:02:09,480 Until last spring, a house stood here. 27 00:02:10,560 --> 00:02:13,120 A home lived in by two brothers, 28 00:02:13,120 --> 00:02:16,360 Jeff and Jeremy Bush, along with their families. 29 00:02:18,600 --> 00:02:20,240 My bedroom was right here. 30 00:02:20,240 --> 00:02:21,640 So, your bedroom was here? 31 00:02:21,640 --> 00:02:23,720 Yeah, my bedroom was right here in the front. 32 00:02:23,720 --> 00:02:26,960 And you walked in through the front door, and there was a living room. 33 00:02:26,960 --> 00:02:29,720 Yeah, so, you went in and then the living room was on the right? 34 00:02:29,720 --> 00:02:32,600 Yeah, living room was on the right, kitchen behind my bedroom, 35 00:02:32,600 --> 00:02:35,680 and then the dining room and then my brother's room. 36 00:02:35,680 --> 00:02:37,920 That was... So it was on the far right-hand side. 37 00:02:37,920 --> 00:02:40,880 And then there was another bedroom, it was Janell's... Oh, right. 38 00:02:40,880 --> 00:02:42,160 It was Janell's bedroom. 39 00:02:43,400 --> 00:02:45,240 It was a normal night, I guess? 40 00:02:45,240 --> 00:02:46,560 It was, it was normal. 41 00:02:46,560 --> 00:02:51,280 I had just got home from work, 42 00:02:51,280 --> 00:02:55,280 come in about ten, 10.30, 43 00:02:55,280 --> 00:02:58,840 and told everybody good night and went to bed. 44 00:02:58,840 --> 00:03:00,880 And then that's when it happened. 45 00:03:02,680 --> 00:03:05,320 My wife turned the light on, and... 46 00:03:07,360 --> 00:03:09,600 ..as I was getting ready to walk in the door, 47 00:03:09,600 --> 00:03:13,880 she turned the light on and all you could see was this big hole. 48 00:03:13,880 --> 00:03:18,120 It sounded like a car hit the house, 49 00:03:18,120 --> 00:03:21,640 but the house didn't move, um, nothing moved. 50 00:03:21,640 --> 00:03:24,640 The walls didn't move, nothing. 51 00:03:24,640 --> 00:03:27,560 Pictures were still hanging on the wall, everything. 52 00:03:27,560 --> 00:03:30,360 It just was a loud crash. 53 00:03:34,480 --> 00:03:36,960 That's a scream I'll never forget. 54 00:03:36,960 --> 00:03:38,240 Um... 55 00:03:42,720 --> 00:03:45,560 He just kept saying, "Help me, somebody help me." 56 00:03:45,560 --> 00:03:49,160 SIREN WAILS 57 00:03:49,160 --> 00:03:53,280 That scream was the last anyone heard of Jeff Bush. 58 00:03:53,280 --> 00:03:55,680 No-one had any idea what was happening, 59 00:03:55,680 --> 00:03:58,360 just that he was in trouble. 60 00:03:58,360 --> 00:03:59,640 It was a 911 call 61 00:03:59,640 --> 00:04:03,840 and it said that a family member had fallen underneath the house. 62 00:04:17,240 --> 00:04:20,720 The first one there was Deputy Duvall. 63 00:04:20,720 --> 00:04:22,120 I guess my first idea 64 00:04:22,120 --> 00:04:25,160 was it may have been, like, an accident-type, 65 00:04:25,160 --> 00:04:27,600 like, somebody was trying to renovate their house 66 00:04:27,600 --> 00:04:29,640 and something happened with the floor, 67 00:04:29,640 --> 00:04:32,440 it was rotten, you know, something of that sort. 68 00:04:32,440 --> 00:04:34,040 He rushed straight in. 69 00:04:34,040 --> 00:04:36,000 He went straight in the house, 70 00:04:36,000 --> 00:04:39,160 and he pulled a couple of people out of the house, 71 00:04:39,160 --> 00:04:40,760 made them get out of the house. 72 00:04:40,760 --> 00:04:44,440 Everybody was screaming and kind of running around. 73 00:04:44,440 --> 00:04:48,320 As soon as I saw them, I knew that it wasn't just somebody 74 00:04:48,320 --> 00:04:52,200 that had fallen into the floor by an accident, you know? 75 00:04:52,200 --> 00:04:54,400 I knew just from their reactions 76 00:04:54,400 --> 00:04:56,640 that it was something a lot more significant. 77 00:04:56,640 --> 00:05:00,080 I looked inside the room. There was nothing. 78 00:05:00,080 --> 00:05:02,640 All you could do was smell fresh dirt. 79 00:05:02,640 --> 00:05:04,560 It took his whole bedroom. 80 00:05:04,560 --> 00:05:08,000 The only piece of concrete that was left was by the door. 81 00:05:08,000 --> 00:05:11,680 It took his bed, his dresser, his TV and everything down in the hole. 82 00:05:11,680 --> 00:05:14,080 The door was open and when I went through the house, 83 00:05:14,080 --> 00:05:16,360 everything looked like it was normal. 84 00:05:16,360 --> 00:05:19,000 You know, the floor was intact and the lights were on, 85 00:05:19,000 --> 00:05:20,520 the power was working. 86 00:05:20,520 --> 00:05:23,480 Nothing out of the ordinary - but I went to the bedroom, 87 00:05:23,480 --> 00:05:26,920 and the door was open, and as soon as I turned and looked in, 88 00:05:26,920 --> 00:05:30,160 there was nothing in the bedroom. It was just a giant hole. 89 00:05:32,040 --> 00:05:36,400 Jeff had been pulled down into the underworld by a sinkhole. 90 00:05:39,160 --> 00:05:42,120 All I could see was the tip of his box spring, 91 00:05:42,120 --> 00:05:45,800 the tip of his bed frame and his mattress, and that was it. 92 00:05:45,800 --> 00:05:47,720 You didn't see Jeff at that point? 93 00:05:47,720 --> 00:05:50,480 Couldn't see Jeff, I thought I... I thought I could hear him 94 00:05:50,480 --> 00:05:53,720 yelling for me to help him. Yeah, and so you just... 95 00:05:53,720 --> 00:05:56,480 I just jumped in there and started digging. 96 00:05:56,480 --> 00:05:58,880 You want to take a minute? 97 00:05:58,880 --> 00:06:03,880 Jeremy was... If you're walking into the actual bedroom, 98 00:06:03,880 --> 00:06:07,840 he's pretty much right there between the door and the centre of the hole. 99 00:06:07,840 --> 00:06:09,760 I started digging by my hands, 100 00:06:09,760 --> 00:06:11,800 and I was yelling and screaming for him, 101 00:06:11,800 --> 00:06:14,640 and yelling for my father-in-law to get a shovel 102 00:06:14,640 --> 00:06:17,280 and a flashlight, so I could see. 103 00:06:17,280 --> 00:06:21,040 So I grabbed hold of his bed and tried moving his bed, 104 00:06:21,040 --> 00:06:22,480 and it wouldn't move 105 00:06:22,480 --> 00:06:26,240 and I broke the bedframe in half trying to get it out. 106 00:06:26,240 --> 00:06:29,280 Then I started... He got me the shovel and I started digging. 107 00:06:29,280 --> 00:06:32,920 And they were real close. You've seen Jeremy, you've seen Jeff. 108 00:06:32,920 --> 00:06:36,560 And they worked together, they did everything together. 109 00:06:36,560 --> 00:06:39,040 They played video games together, they... 110 00:06:40,360 --> 00:06:43,480 They were... They were brothers, they were tight brothers. 111 00:06:46,480 --> 00:06:49,120 Jeff Bush had been swallowed alive, 112 00:06:49,120 --> 00:06:53,240 and the sinkhole came close to swallowing his brother, Jeremy, too. 113 00:06:55,360 --> 00:06:58,000 The ground was still falling as I was in the hole 114 00:06:58,000 --> 00:07:02,200 and the concrete was moving and breaking, still. 115 00:07:02,200 --> 00:07:04,240 So, you were on your way down? I was on my way down. 116 00:07:04,240 --> 00:07:07,240 And you didn't even notice it? I wasn't paying attention to it at all. 117 00:07:07,240 --> 00:07:09,680 I was just trying to get my brother out. 118 00:07:09,680 --> 00:07:13,560 If you can imagine an hourglass, the funnel inside of an hourglass, 119 00:07:13,560 --> 00:07:15,800 you've got the deeper portion, 120 00:07:15,800 --> 00:07:18,680 then you've got the out... the outermost wall. 121 00:07:18,680 --> 00:07:22,080 Jeremy was in the middle and while it was sinking, 122 00:07:22,080 --> 00:07:23,640 it was also expanding out 123 00:07:23,640 --> 00:07:27,960 because everything on the outside was filling in the void. 124 00:07:27,960 --> 00:07:30,520 I know I didn't want to come out the hole, 125 00:07:30,520 --> 00:07:33,800 I wanted to pull the deputy in there with me to help me dig him out, 126 00:07:33,800 --> 00:07:35,960 because nobody was helping. Of course. 127 00:07:40,400 --> 00:07:44,120 Those who saw it forming will never forget it. 128 00:07:44,120 --> 00:07:45,840 When everybody asks me about it, 129 00:07:45,840 --> 00:07:48,720 I tell them it's like this thing was alive, 130 00:07:48,720 --> 00:07:51,840 and when I say that it was eating, it literally... 131 00:07:52,960 --> 00:07:56,000 Things, the furniture, it was still sinking, 132 00:07:56,000 --> 00:07:58,240 it was still going into the ground. 133 00:07:58,240 --> 00:08:03,320 And, you know, it was kind of, kind of turning, moving around, 134 00:08:03,320 --> 00:08:04,760 making noises, you know, 135 00:08:04,760 --> 00:08:09,600 almost like a growl, and it just... Like something was alive. 136 00:08:15,280 --> 00:08:17,680 Jeremy was seconds from being sucked down 137 00:08:17,680 --> 00:08:20,160 to the same terrible fate as his brother 138 00:08:20,160 --> 00:08:22,000 when Deputy Duvall saved him. 139 00:08:23,440 --> 00:08:26,240 He grabbed me by my arm and snatched me out of the hole, 140 00:08:26,240 --> 00:08:30,400 and that was the last time that anybody went back in the house. 141 00:08:40,880 --> 00:08:45,360 When the sun came up, everything seemed normal from the outside. 142 00:08:45,360 --> 00:08:48,000 All anyone knew was that Jeff Bush was gone, 143 00:08:48,000 --> 00:08:50,240 his body never to be discovered. 144 00:08:55,080 --> 00:08:58,280 I found meeting Jeremy and hearing what happened to his brother 145 00:08:58,280 --> 00:09:01,320 deeply disturbing and unsettling. 146 00:09:02,880 --> 00:09:05,440 Such a horrible thought, isn't it? 147 00:09:05,440 --> 00:09:10,160 That idea of the ground opening up and literally swallowing you alive, 148 00:09:10,160 --> 00:09:13,000 it's just... It's the stuff of nightmares. 149 00:09:17,880 --> 00:09:22,320 My science, geology, tries to give answers to why things happen 150 00:09:22,320 --> 00:09:24,440 and hopefully save lives. 151 00:09:26,080 --> 00:09:30,040 And one of the reasons I've become so interested in sinkholes right now 152 00:09:30,040 --> 00:09:32,520 is that I've noticed more of them in the news. 153 00:09:39,520 --> 00:09:41,240 In the last few years, 154 00:09:41,240 --> 00:09:44,080 they have been captured as far afield as China... 155 00:09:46,920 --> 00:09:48,320 ..and Guatemala. 156 00:09:50,240 --> 00:09:53,080 They create the fear that the ground beneath our feet 157 00:09:53,080 --> 00:09:56,080 could open up into an unseen world at any time. 158 00:10:00,720 --> 00:10:03,280 Incredibly, this young girl survived. 159 00:10:07,640 --> 00:10:09,080 But it's in Florida 160 00:10:09,080 --> 00:10:12,280 where the fear of dropping into the underworld is greatest. 161 00:10:16,160 --> 00:10:18,200 A few months after Jeff Bush died, 162 00:10:18,200 --> 00:10:21,480 over 100 people were saved from this resort complex 163 00:10:21,480 --> 00:10:23,920 near Disney World just before it collapsed. 164 00:10:30,200 --> 00:10:34,320 The dubious honour of being called the sinkhole capital of the world 165 00:10:34,320 --> 00:10:38,040 costs the state of Florida hundreds of millions of dollars a year. 166 00:10:39,440 --> 00:10:41,640 So at least lawyers are getting something out of it. 167 00:10:41,640 --> 00:10:42,800 Look at that. 168 00:10:42,800 --> 00:10:45,640 As soon as homeowners see some kind of crack in their house, 169 00:10:45,640 --> 00:10:48,480 they're encouraged to phone these sinkhole attorneys. 170 00:10:49,960 --> 00:10:51,360 And they ply their trade 171 00:10:51,360 --> 00:10:54,360 up and down the highway here with all their billboards. 172 00:11:03,320 --> 00:11:07,320 I've never seen a sinkhole before, not up close, 173 00:11:07,320 --> 00:11:10,800 but to really understand them you can't just look at the surface. 174 00:11:10,800 --> 00:11:14,200 You have to see what's happening underneath. 175 00:11:14,200 --> 00:11:18,920 If you really want to know what the Florida underworld is like, you've got to descend into it. 176 00:11:18,920 --> 00:11:20,360 On the rope! 177 00:11:29,680 --> 00:11:32,520 There's one thing I could tell straightaway - 178 00:11:32,520 --> 00:11:35,400 the rock I'm descending through is very familiar. 179 00:11:37,400 --> 00:11:40,040 It's one of the most common rock types in the world 180 00:11:40,040 --> 00:11:43,920 and obviously one of the most useful - it's a raw ingredient for cement. 181 00:11:43,920 --> 00:11:48,600 But it's also the maker of these fantastic subterranean worlds. 182 00:11:53,800 --> 00:11:57,520 If you were walking above, you'd never know this was here. 183 00:11:59,440 --> 00:12:03,400 A great void, taking shape beneath the surface of the earth. 184 00:12:06,520 --> 00:12:11,560 Professor Jason Polk has been trying to understand what it is that makes them collapse. 185 00:12:12,960 --> 00:12:18,880 He estimates that the caves started forming millions of years ago. 186 00:12:18,880 --> 00:12:22,720 That's the last kind of part of it, breaching to the surface? 187 00:12:22,720 --> 00:12:25,160 Eventually the rock becomes so thin 188 00:12:25,160 --> 00:12:28,560 that a large collapse can occur instantaneously. 189 00:12:28,560 --> 00:12:31,440 And a lot of the sinkholes you see in Florida where 190 00:12:31,440 --> 00:12:35,160 you have those instant collapses are where it's thin rock and thick soil. 191 00:12:35,160 --> 00:12:39,560 So what's your best bet about how old that collapse was, when did daylight come in here? 192 00:12:39,560 --> 00:12:42,760 Best guess is from work we've done with the sediments where 193 00:12:42,760 --> 00:12:45,960 we've done radiocarbon dating to actually see how old these sediments are, 194 00:12:45,960 --> 00:12:48,720 and we know that they are at least 10,000 years old, 195 00:12:48,720 --> 00:12:52,800 so that collapse is probably 10,000 years old at a minimum. Yeah. 196 00:12:59,040 --> 00:13:02,400 It may look like the sinkhole's dead, but it's anything but. 197 00:13:05,720 --> 00:13:10,240 Ever since the roof fell in, the cave has been filling with sediment. 198 00:13:10,240 --> 00:13:14,920 But then three years ago, that sediment disappeared. 199 00:13:14,920 --> 00:13:18,160 So 2010 then, it was up to there, is that right? 200 00:13:18,160 --> 00:13:20,000 You would have been under sediment. 201 00:13:20,000 --> 00:13:22,840 And just within minutes, hours - straight down? 202 00:13:22,840 --> 00:13:26,080 Just almost immediately. And where, down...down here? 203 00:13:26,080 --> 00:13:29,680 Straight down here in this hole, which is the pathway to some 204 00:13:29,680 --> 00:13:33,320 unknown void below where all of this sediment is washed down and continues to wash down. 205 00:13:35,040 --> 00:13:39,280 You've not been down there? Never been in there, didn't even know it existed. God! 206 00:13:39,280 --> 00:13:41,920 So do you reckon it's...? I mean, are you slightly freaked out 207 00:13:41,920 --> 00:13:44,000 that underneath we know there's a big hole? 208 00:13:44,000 --> 00:13:48,240 It feels solid but we know that if we'd have been here a few years ago when this happened, we'd be... 209 00:13:48,240 --> 00:13:51,440 We'd be down there. We'd be down below in the unknown. 210 00:13:51,440 --> 00:13:54,080 Wow, that's a bit scary. We should go. 211 00:13:54,080 --> 00:13:55,680 I don't like that, actually. 212 00:13:55,680 --> 00:14:00,200 That gaping hole showed how alive this sinkhole still is. 213 00:14:00,200 --> 00:14:02,240 I was very glad to leave. 214 00:14:14,640 --> 00:14:17,480 I've been told that if I want to get a sense of just what 215 00:14:17,480 --> 00:14:21,560 a problem sinkholes really are here, there's somewhere I should go. 216 00:14:23,360 --> 00:14:25,520 This estate is just north of Tampa. 217 00:14:26,960 --> 00:14:30,280 In many ways, it's a pretty unremarkable place. 218 00:14:32,280 --> 00:14:36,400 Full of retirees chasing the dream of all year round sun. 219 00:14:36,400 --> 00:14:39,440 It all looks so perfect, doesn't it? I mean, look at it. 220 00:14:39,440 --> 00:14:42,400 But behind the facade, virtually all of these houses have got 221 00:14:42,400 --> 00:14:45,520 structural problems and cracks and got people going to 222 00:14:45,520 --> 00:14:48,560 bed at night really not sure if those sounds that they're 223 00:14:48,560 --> 00:14:52,080 hearing is going to be another hole appearing underneath them. 224 00:14:57,080 --> 00:15:00,720 So instead of spending their time making extensions and patios... 225 00:15:03,840 --> 00:15:07,840 ..the builders round here are pouring hundreds of tonnes of cement beneath 226 00:15:07,840 --> 00:15:11,400 the foundations of these houses in an attempt to stabilise them. 227 00:15:20,840 --> 00:15:24,520 'I went to visit one of the long-standing residents, Darlene Denaro.' 228 00:15:26,120 --> 00:15:29,320 So how many of these houses would you say have had problems 229 00:15:29,320 --> 00:15:32,640 or are getting problems now? 230 00:15:32,640 --> 00:15:37,320 Erm...well, she has one, and in one down there on that side... 231 00:15:37,320 --> 00:15:39,520 By the white car? By the white car, 232 00:15:39,520 --> 00:15:44,320 and then across the street there's one that was fixed. Right. 233 00:15:44,320 --> 00:15:47,320 Yeah, then this one and this one, 234 00:15:47,320 --> 00:15:51,560 and then she suspects she has a sinkhole. 235 00:15:51,560 --> 00:15:53,960 Which one, which one, the one with the green car? 236 00:15:53,960 --> 00:15:57,000 No, on the other side. Other side of that? 237 00:15:57,000 --> 00:16:00,480 Yeah, and then around the corner, where Louis lives, 238 00:16:00,480 --> 00:16:04,720 he lives on this side right here and he had, they had... 239 00:16:04,720 --> 00:16:10,000 It's one, two, three and then one down here, he had a humungous one. 240 00:16:10,000 --> 00:16:13,760 I think he had pretty close to 70 truck loads. 241 00:16:13,760 --> 00:16:16,920 So you're kind of in the centre, really, of this... Sorry, well, 242 00:16:16,920 --> 00:16:19,640 yeah, I was thinking, I mean, that... 243 00:16:19,640 --> 00:16:23,000 I feel like I'm in... Yeah, I'm in the centre. 244 00:16:27,280 --> 00:16:31,560 For decades, new housing has been springing up all across Florida. 245 00:16:33,200 --> 00:16:35,480 Built on what was once rough farmland, 246 00:16:35,480 --> 00:16:38,760 they seemed to offer a golden future for their new inhabitants. 247 00:16:41,280 --> 00:16:43,800 But now I get the sense that many of them 248 00:16:43,800 --> 00:16:47,880 are gripped by a collective fear of what might lie beneath. 249 00:16:49,040 --> 00:16:53,080 When you moved here, was anything mentioned about sinkholes here? Nothing. 250 00:16:53,080 --> 00:16:57,240 What was it before? Farmland, wet farmland, swampland. 251 00:16:57,240 --> 00:17:00,480 Everybody who's... The old Florida people that had been 252 00:17:00,480 --> 00:17:05,080 here for years, it was a very wet, soggy, swamp piece of property. 253 00:17:05,080 --> 00:17:07,080 We would have never moved here, never. 254 00:17:07,080 --> 00:17:08,800 If you'd known. No. 255 00:17:18,600 --> 00:17:21,920 What's happening on this estate is not that unusual - 256 00:17:21,920 --> 00:17:24,760 sinkholes ruining the American Dream. 257 00:17:26,800 --> 00:17:28,920 Look at this. Here we've got one. 258 00:17:30,640 --> 00:17:32,960 For a lot of people, this is reality now, 259 00:17:32,960 --> 00:17:36,200 trying to fill in this huge void underneath their houses. 260 00:17:40,680 --> 00:17:43,640 I detected a real fear on this estate. 261 00:17:43,640 --> 00:17:48,360 After all, Jeff Bush lived just an hour down the road. 262 00:17:48,360 --> 00:17:50,520 But how justified are those fears? 263 00:18:01,680 --> 00:18:05,800 It's very hard to get a sense of how many sinkholes there are in this state. 264 00:18:12,760 --> 00:18:15,560 And just what a threat they are to the people living here. 265 00:18:22,880 --> 00:18:26,520 So I'm going to try and get a rather higher perspective on the problem. 266 00:18:26,520 --> 00:18:28,640 RADIO CHATTER 267 00:18:39,480 --> 00:18:42,640 It's going to be so good to get up top because, I mean, to be 268 00:18:42,640 --> 00:18:45,400 honest, on the ground, it's quite a tedious landscape. 269 00:18:45,400 --> 00:18:49,680 It's very flat, there's lots of trees and tarmac that obscure 270 00:18:49,680 --> 00:18:53,360 the views, but I think that from here, everything will become clear. 271 00:19:01,640 --> 00:19:04,080 So we're going to go up about 500 feet, is that right? 272 00:19:05,400 --> 00:19:07,240 Above sea level. Right. 273 00:19:09,960 --> 00:19:14,040 What I'm looking for is evidence of ancient sinkholes, 274 00:19:14,040 --> 00:19:17,680 depressions in the land that have formed into lakes. 275 00:19:17,680 --> 00:19:20,200 You can start to see some now. You see that over there? 276 00:19:29,360 --> 00:19:31,720 Just a pockmark, a series of little lakes. 277 00:19:33,520 --> 00:19:37,200 Like the whole place now is just lake land, everywhere you can see. 278 00:19:37,200 --> 00:19:39,480 It's just little pockets, like just here. 279 00:19:43,760 --> 00:19:47,760 Looks like the whole place is a giant golf course with kind of water hazards. 280 00:19:47,760 --> 00:19:49,800 This is pretty much Florida, I guess. 281 00:19:52,040 --> 00:19:54,080 Each one of those lakes beneath us 282 00:19:54,080 --> 00:19:56,920 started as a hole into a limestone cavern beneath. 283 00:20:06,080 --> 00:20:09,080 It's quite interesting, there's this intricate anatomy to them. 284 00:20:09,080 --> 00:20:11,720 Some of them are just, just perfect circles, 285 00:20:11,720 --> 00:20:14,000 and that's just one sinkhole, but others you can see, 286 00:20:14,000 --> 00:20:16,600 it's like four or five have all joined together. 287 00:20:19,360 --> 00:20:22,520 So the whole thing is just pockmarked as far as you can see, 288 00:20:22,520 --> 00:20:25,600 filled with water, so you get these ornamental water features 289 00:20:25,600 --> 00:20:28,000 that people build their houses and jetties around. 290 00:20:28,000 --> 00:20:29,800 They're really sought-after. 291 00:20:35,520 --> 00:20:39,040 But the thing is, potentially, they're lethal. I mean, these things can open up 292 00:20:39,040 --> 00:20:41,280 and you're left with a hole in the ground. 293 00:20:43,400 --> 00:20:47,400 It's reckoned there are several thousand of these sinkhole 294 00:20:47,400 --> 00:20:51,240 lakes across the state of Florida, several thousand of them! 295 00:20:51,240 --> 00:20:54,000 And that's just a part of it. 296 00:20:54,000 --> 00:20:58,200 I tried to get a figure on just how many sinkholes there are in the state. 297 00:20:58,200 --> 00:21:01,520 One county claims to have over 6,000 of them, 298 00:21:01,520 --> 00:21:04,320 but the truth is, no-one really knows. 299 00:21:04,320 --> 00:21:06,560 They've literally stopped counting. 300 00:21:25,440 --> 00:21:27,520 To understand why, 301 00:21:27,520 --> 00:21:31,680 you have to travel back to the very origins of the state itself. 302 00:21:33,840 --> 00:21:37,920 Head a couple of miles out to sea and you can observe the whole process beginning. 303 00:21:46,000 --> 00:21:48,200 Tens of millions of years ago, 304 00:21:48,200 --> 00:21:53,360 modern Florida emerged as shallow seas just like this receded. 305 00:21:56,560 --> 00:21:59,760 Warm seas that once teemed with marine life. 306 00:22:13,000 --> 00:22:16,240 Quite nice, nice temperature. 307 00:22:16,240 --> 00:22:18,600 No need for a snorkel and flippers. 308 00:22:18,600 --> 00:22:23,440 It's amazingly shallow, even two miles off the shore. 309 00:22:23,440 --> 00:22:25,720 For a wee walk in the Florida Keys. 310 00:22:28,920 --> 00:22:32,640 What I'm standing on is the remains of the marine life that 311 00:22:32,640 --> 00:22:36,280 inhabited this warm shallow coastline. 312 00:22:36,280 --> 00:22:38,280 And the remains of those creatures 313 00:22:38,280 --> 00:22:40,800 form a mud made from carbonates. 314 00:22:41,840 --> 00:22:44,480 When those creatures die and decompose, 315 00:22:44,480 --> 00:22:47,680 they turn into this, carbonate mud. 316 00:22:47,680 --> 00:22:50,640 It's amazing to think this stuff is just the smashed-up 317 00:22:50,640 --> 00:22:53,640 hard parts of millions of sea creatures. 318 00:22:53,640 --> 00:22:56,320 And yet this is Florida in the making. 319 00:22:56,320 --> 00:22:58,920 Over hundreds of thousands of years it gets compressed 320 00:22:58,920 --> 00:23:03,200 into limestone, the rock that virtually all of Florida is made of. 321 00:23:10,640 --> 00:23:15,360 An entire peninsula, around 500 miles north to south 322 00:23:15,360 --> 00:23:18,760 and over 160 east to west. 323 00:23:18,760 --> 00:23:22,760 And almost every inch of that is made of marine organisms. 324 00:23:22,760 --> 00:23:25,120 And that's the key to sinkholes, really, 325 00:23:25,120 --> 00:23:28,560 because what can be created can also be destroyed. 326 00:23:46,440 --> 00:23:49,880 This is the bedrock of Florida stripped bare - 327 00:23:49,880 --> 00:23:52,120 an old quarry at Windley Key. 328 00:24:01,000 --> 00:24:04,520 If this looks solid and unchanging, it isn't. 329 00:24:04,520 --> 00:24:06,520 CREAKING 330 00:24:08,040 --> 00:24:12,240 Something has been eating it away and still is. 331 00:24:19,240 --> 00:24:22,600 The thing is, everywhere you look, this rock is being destroyed. 332 00:24:22,600 --> 00:24:24,400 I mean, look at this bit here. 333 00:24:24,400 --> 00:24:26,800 It's been eroded away. 334 00:24:26,800 --> 00:24:29,480 These are kind of miniature sinkholes, really. 335 00:24:33,360 --> 00:24:35,800 And what's created them is just rain. 336 00:24:35,800 --> 00:24:39,280 The rain is falling down and dissolving them away. 337 00:24:41,280 --> 00:24:45,360 But to eat away the limestone, that rain has to change. 338 00:24:45,360 --> 00:24:49,080 Pure water has a pH of around about seven, 339 00:24:49,080 --> 00:24:51,640 but as it falls through the atmosphere it picks up 340 00:24:51,640 --> 00:24:55,160 carbon dioxide molecules that turns it into a very weak acid, 341 00:24:55,160 --> 00:24:56,520 carbonic acid. 342 00:24:56,520 --> 00:24:58,760 And the other thing is that if it hits, here, 343 00:24:58,760 --> 00:25:00,560 rotting vegetation and soil, 344 00:25:00,560 --> 00:25:02,800 then that pH drops even more. 345 00:25:02,800 --> 00:25:06,920 So, look at this. This is a pH meter and it's reading seven. 346 00:25:06,920 --> 00:25:10,120 Now, if I stick it into this soil here, 347 00:25:10,120 --> 00:25:13,200 it's gone down - 6.6, 5.3. 348 00:25:13,200 --> 00:25:16,640 So that is really quite dramatically more acidic. 349 00:25:16,640 --> 00:25:20,640 It's that acidity over thousands, tens of thousands of years, 350 00:25:20,640 --> 00:25:23,520 that basically eats out those huge caves. 351 00:25:23,520 --> 00:25:26,600 And if you don't believe me that acid can dissolve away rock, 352 00:25:26,600 --> 00:25:28,000 I've got a little bit of acid. 353 00:25:28,000 --> 00:25:30,040 It's hydrochloric acid, but it's quite dilute. 354 00:25:30,040 --> 00:25:33,080 Look, if I pour it on my skin, it does nothing. 355 00:25:34,760 --> 00:25:37,960 But if I pour it on this fossil coral... 356 00:25:37,960 --> 00:25:40,160 look at that. 357 00:25:40,160 --> 00:25:42,040 It's just going crazy. 358 00:25:44,120 --> 00:25:45,880 One moment you've got calcium carbonate, 359 00:25:45,880 --> 00:25:50,280 next moment it's all fizzed back to carbon dioxide. 360 00:25:50,280 --> 00:25:54,400 And it's this process that's very slowly dissolving the whole state. 361 00:25:57,880 --> 00:25:59,480 One of the things that drew me 362 00:25:59,480 --> 00:26:02,800 to geology is how it makes you see the certainties 363 00:26:02,800 --> 00:26:07,200 of the world we've created, the human planet, rather differently. 364 00:26:07,200 --> 00:26:11,280 There are almost 20 million people living in Florida 365 00:26:11,280 --> 00:26:13,280 and the population's growing rapidly. 366 00:26:14,520 --> 00:26:17,440 Driving along the streets of somewhere like Miami, you feel 367 00:26:17,440 --> 00:26:21,120 as if you're in one of the safest, most modern places in the world. 368 00:26:24,880 --> 00:26:28,600 It's all built on rocks that are being dissolved by water. 369 00:26:28,600 --> 00:26:32,600 Every drop that falls from the sky, 370 00:26:32,600 --> 00:26:36,960 every drop that sinks through the ground, is turning to acid, 371 00:26:36,960 --> 00:26:42,720 and that acid is very slowly dissolving the whole state. 372 00:26:46,000 --> 00:26:49,960 To witness it in action, you have to leave the urban sprawl 373 00:26:49,960 --> 00:26:52,320 and head out into the old Florida, 374 00:26:52,320 --> 00:26:55,720 the Florida that existed before people ever set foot here. 375 00:26:59,080 --> 00:27:01,000 Step just a few feet from the freeway 376 00:27:01,000 --> 00:27:04,080 and you're into this primeval land of swamp and alligator. 377 00:27:07,520 --> 00:27:11,600 It's a world I'm entering in search of something rather special. 378 00:27:28,840 --> 00:27:31,840 I'm off to meet up with a remarkable team of explorers 379 00:27:31,840 --> 00:27:33,880 who will really be able to show me 380 00:27:33,880 --> 00:27:36,520 why there are so many sinkholes in Florida. 381 00:27:42,680 --> 00:27:44,720 This is Peacock Springs. 382 00:27:47,200 --> 00:27:49,000 It's an alien place. 383 00:27:52,920 --> 00:27:55,680 It's a part of Florida that most tourists, 384 00:27:55,680 --> 00:27:58,120 even most residents, never see. 385 00:28:02,280 --> 00:28:03,680 It's like a lost world. 386 00:28:16,600 --> 00:28:20,080 The water is just so clear. 387 00:28:20,080 --> 00:28:22,080 And so it should be, really, 388 00:28:22,080 --> 00:28:25,800 because this is Florida's lifeblood. 389 00:28:25,800 --> 00:28:29,400 Something like 95% of the state's ground water comes through 390 00:28:29,400 --> 00:28:31,000 springs like this. 391 00:28:35,880 --> 00:28:39,120 And what's so special about this place is that beneath me 392 00:28:39,120 --> 00:28:42,400 is a massive flooded cave complex, 393 00:28:42,400 --> 00:28:45,120 the so-called Floridan aquifer, 394 00:28:45,120 --> 00:28:47,360 through which all this water's flowed. 395 00:28:49,080 --> 00:28:52,960 And it's home to some of the most intriguing sinkholes in the state. 396 00:28:54,760 --> 00:28:56,680 Hey! Hi! How's it going? 397 00:28:56,680 --> 00:28:59,680 You have picked the most beautiful spot. 398 00:28:59,680 --> 00:29:02,720 Yeah, it's a remarkable area. Fantastic. 399 00:29:02,720 --> 00:29:05,960 'Jarrod Jablonski and his team are among the most experienced 400 00:29:05,960 --> 00:29:09,240 'cave divers in the world, and they've been exploring 401 00:29:09,240 --> 00:29:11,640 'cave systems like these for decades.' 402 00:29:13,080 --> 00:29:15,520 Ah! That's better. Dry land. 403 00:29:15,520 --> 00:29:17,920 It's quite deep in there, though. I hadn't realised. 404 00:29:17,920 --> 00:29:20,600 Yeah, the water's actually a pretty good level. 405 00:29:20,600 --> 00:29:23,040 It varies a lot, depending on the drought conditions, 406 00:29:23,040 --> 00:29:24,640 how much rain we're getting. 407 00:29:25,680 --> 00:29:28,600 'What most people would find terrifying, 408 00:29:28,600 --> 00:29:31,760 'these divers find magical.' 409 00:29:31,760 --> 00:29:33,760 You've got this hidden world that no-one else 410 00:29:33,760 --> 00:29:35,240 but a few of you guys know about. 411 00:29:35,240 --> 00:29:38,040 Yeah, very much. We kind of enjoy that every now and then. 412 00:29:38,040 --> 00:29:40,520 You go down and you're looking around and it's just you 413 00:29:40,520 --> 00:29:43,720 and you start thinking about how few people in the world get that 414 00:29:43,720 --> 00:29:45,080 really special experience. 415 00:29:49,840 --> 00:29:53,440 'Even though Jarrod and his team are really experienced, 416 00:29:53,440 --> 00:29:57,160 'what they're about to do is still incredibly dangerous.' 417 00:29:59,080 --> 00:30:02,320 How many people have been killed, do you think, in these caves diving? 418 00:30:02,320 --> 00:30:04,680 Probably somewhere in the neighbourhood of 500 people 419 00:30:04,680 --> 00:30:07,400 have lost their lives exploring Florida caves. 420 00:30:07,400 --> 00:30:10,760 Most of those, certainly by a great majority, especially in the early 421 00:30:10,760 --> 00:30:14,200 years, were untrained open water divers, so really a very bad recipe. 422 00:30:14,200 --> 00:30:16,440 So they thought it was the same environment as the sea? 423 00:30:16,440 --> 00:30:19,080 Yeah, didn't know. Swim around in a beautiful place like this - 424 00:30:19,080 --> 00:30:21,320 as you said, it's beautiful and very benign looking. 425 00:30:21,320 --> 00:30:23,840 Then you go into the cave, which also looks at first benign, 426 00:30:23,840 --> 00:30:26,800 and then, if you don't know what you're doing, you can kick up the 427 00:30:26,800 --> 00:30:29,760 bottom conditions and you don't have a guideline, get lost pretty easily. 428 00:30:32,400 --> 00:30:37,160 'The entrance to this hidden, deadly Florida was just below us 429 00:30:37,160 --> 00:30:39,800 'and, whilst cave diving is a step too far for me, 430 00:30:39,800 --> 00:30:43,800 'I was keen just to look into its jaws.' 431 00:30:43,800 --> 00:30:46,520 So where's the entrance then? We're going to go down right here 432 00:30:46,520 --> 00:30:50,360 and we're going to go right in this way, which is going to descend down. 433 00:30:50,360 --> 00:30:52,560 We'll have about 6m deep and then we'll 434 00:30:52,560 --> 00:30:55,600 descend down to depth which will be about 20m deep. 435 00:30:55,600 --> 00:30:57,480 And you're going to head in that direction? 436 00:30:57,480 --> 00:31:00,320 In this direction towards a sinkhole called Pothole 437 00:31:00,320 --> 00:31:02,760 and then the next sinkhole, Olsen. 438 00:31:02,760 --> 00:31:05,880 There's a series of sinkholes that you can access through this conduit. 439 00:31:05,880 --> 00:31:08,360 All right, well, I'll see you off the premises, then. 440 00:31:08,360 --> 00:31:10,480 All right. Excuse me! 441 00:31:10,480 --> 00:31:13,120 I'll see how long I can hold my breath. 442 00:31:13,120 --> 00:31:16,560 I like it. Don't take me with you. 443 00:31:16,560 --> 00:31:18,840 All set? Good stuff! 444 00:31:46,520 --> 00:31:49,000 Well, they've gone. 445 00:31:49,000 --> 00:31:53,120 I can just see the last of them disappearing into the entrance. 446 00:31:53,120 --> 00:31:54,960 But, to be honest, it's not for me, 447 00:31:54,960 --> 00:31:57,600 so I'm going to follow them on dry land. 448 00:31:58,920 --> 00:32:00,440 As I headed ashore, 449 00:32:00,440 --> 00:32:04,080 Jarrod and team swam into the very throat of the underworld. 450 00:32:16,480 --> 00:32:18,520 For all their beauty, 451 00:32:18,520 --> 00:32:20,160 these labyrinths are lethal. 452 00:32:21,960 --> 00:32:25,280 All a diver has to do is kick up the sediment with a careless 453 00:32:25,280 --> 00:32:29,080 flick of a fin and the visibility will reduce to zero. 454 00:32:30,680 --> 00:32:33,560 That's how most people lose their lives down here. 455 00:32:39,000 --> 00:32:42,880 What I find amazing about these caves is how extensive they are. 456 00:32:46,000 --> 00:32:49,400 Divers have explored over 10km of them, 457 00:32:49,400 --> 00:32:52,840 but the caves' conduits and pore spaces of this aquifer 458 00:32:52,840 --> 00:32:55,880 stretch from one end of the state to the other. 459 00:33:04,640 --> 00:33:07,840 It's weird to think that they're right beneath my feet. 460 00:33:07,840 --> 00:33:10,320 Apparently, the first little bit's really tortuous, 461 00:33:10,320 --> 00:33:13,480 so if they manage to squeeze their way through that, then, according 462 00:33:13,480 --> 00:33:17,600 to this map, we should get the first indication of progress just up here. 463 00:33:21,360 --> 00:33:24,160 What I'm looking for is a small sinkhole - 464 00:33:24,160 --> 00:33:27,280 a place where the expanding cave has reached the point where 465 00:33:27,280 --> 00:33:30,640 its roof has failed, allowing the soils to fall down, 466 00:33:30,640 --> 00:33:34,080 creating a link between the worlds above and below. 467 00:33:40,160 --> 00:33:43,240 It's a sobering thought that those caves 468 00:33:43,240 --> 00:33:45,760 are expanding in all directions. 469 00:33:49,000 --> 00:33:50,600 And that's because all it takes 470 00:33:50,600 --> 00:33:54,080 to make the limestone dissolve is water, 471 00:33:54,080 --> 00:33:56,240 and there's plenty of that. 472 00:34:04,680 --> 00:34:07,080 So this is it. 473 00:34:14,720 --> 00:34:16,960 So this is Pothole sink, then. 474 00:34:16,960 --> 00:34:20,200 We've come from just about two minutes' walk away, 475 00:34:20,200 --> 00:34:22,840 from this Peacock Spring where the guys went in. 476 00:34:22,840 --> 00:34:25,840 And, at one point, the limestone would have been across, like this, 477 00:34:25,840 --> 00:34:28,680 but then what's happened is this bit's been dissolved down and then 478 00:34:28,680 --> 00:34:32,200 the wear of the soil at some point - maybe, I don't know, 10,000 years, 479 00:34:32,200 --> 00:34:35,600 20,000 years ago - the whole thing's just caved in on itself, 480 00:34:35,600 --> 00:34:38,880 collapsed, creating this hole, and you've got this. 481 00:34:38,880 --> 00:34:40,920 This is essentially a conduit. 482 00:34:40,920 --> 00:34:44,160 It's maybe 40, 50 foot down below. 483 00:34:44,160 --> 00:34:48,640 So, somewhere down there the guys are swimming past and supposedly 484 00:34:48,640 --> 00:34:51,800 what happens is you're going to see the bubbles as they go past. 485 00:34:51,800 --> 00:34:53,520 That's what we're waiting for. 486 00:35:05,280 --> 00:35:08,640 This is a nursery ground for sinkholes, 487 00:35:08,640 --> 00:35:13,200 the rock dissolving at a rate of around 4cm every thousand years. 488 00:35:16,880 --> 00:35:18,680 The void's getting bigger 489 00:35:18,680 --> 00:35:22,560 and ultimately the soil above falling into them, 490 00:35:22,560 --> 00:35:24,880 creating yet another sinkhole. 491 00:35:45,680 --> 00:35:47,120 There, there we go! 492 00:35:48,360 --> 00:35:51,000 So that means they're now right beneath us. 493 00:35:51,000 --> 00:35:53,440 The air's coming up, so that means they're safe so far. 494 00:35:53,440 --> 00:35:56,280 So, the thing is, they've got another hour to go before they can 495 00:35:56,280 --> 00:36:00,360 actually surface properly and have proper fresh air. 496 00:36:00,360 --> 00:36:03,520 At least they're safe. So far, so good. 497 00:36:03,520 --> 00:36:05,400 Now, onto the next bit. 498 00:36:09,960 --> 00:36:13,120 Dissolving limestone like this is known as karst, 499 00:36:13,120 --> 00:36:16,200 and it's not just confined to Florida. 500 00:36:19,520 --> 00:36:22,440 Limestone is common all over the world. 501 00:36:24,280 --> 00:36:26,360 There are pockets of it in the UK, 502 00:36:26,360 --> 00:36:29,160 but here sinkholes rarely make the news. 503 00:36:35,280 --> 00:36:39,560 What's special about Florida is the extent of the limestone 504 00:36:39,560 --> 00:36:42,080 and just how big the sinkholes are. 505 00:36:44,240 --> 00:36:47,040 You know, it's the kind of place that makes you contemplate 506 00:36:47,040 --> 00:36:49,720 Florida in a whole different way. 507 00:36:49,720 --> 00:36:52,560 If I hadn't have known there was a team of divers down there, 508 00:36:52,560 --> 00:36:54,680 this would just be another pond 509 00:36:54,680 --> 00:36:56,800 and another little patch of wood, 510 00:36:56,800 --> 00:36:59,960 but actually it's a gateway to the underworld, 511 00:36:59,960 --> 00:37:04,200 an underworld that stretches the length and breadth of Florida 512 00:37:04,200 --> 00:37:06,360 and an underworld that's killed 513 00:37:06,360 --> 00:37:08,200 and will kill again. 514 00:37:12,680 --> 00:37:16,920 So, at any point, I'm hoping to see this burst of bubbles 515 00:37:16,920 --> 00:37:18,760 and that'll be them, safe. 516 00:37:24,920 --> 00:37:26,240 Oh, yes! 517 00:37:26,240 --> 00:37:27,480 That must be them, look. 518 00:37:29,440 --> 00:37:33,000 Look at this. Yeah, here they come. Here they come. 519 00:37:33,000 --> 00:37:35,200 What a beautiful spot to come out, as well. 520 00:37:38,760 --> 00:37:41,120 There's something quite elegant about it. 521 00:37:46,760 --> 00:37:50,400 Really big sinkholes happen only rarely, 522 00:37:50,400 --> 00:37:53,600 but when they do, they make quite an impact. 523 00:38:00,280 --> 00:38:04,880 On the 8th of May 1981, the residents of Winter Park, 524 00:38:04,880 --> 00:38:07,000 close to Orlando, witnessed this. 525 00:38:11,960 --> 00:38:15,040 What started as a small hole soon developed into 526 00:38:15,040 --> 00:38:17,440 a 13m deep monster, 527 00:38:17,440 --> 00:38:20,080 some hundred metres wide. 528 00:38:20,080 --> 00:38:22,760 We watched a house slide in, we watched eight or nine cars 529 00:38:22,760 --> 00:38:25,160 slide in, we watched the swimming pool slide in. 530 00:38:25,160 --> 00:38:28,040 And you just sit there and watch it and you're powerless to help. 531 00:38:29,840 --> 00:38:31,840 Any thoughts about making it a lake? 532 00:38:31,840 --> 00:38:34,520 It will be a lake. We've already found that out yesterday. 533 00:38:34,520 --> 00:38:37,400 The engineers and that sort of people said it will be a lake and 534 00:38:37,400 --> 00:38:39,240 there's nothing we can do about that, 535 00:38:39,240 --> 00:38:41,200 so we just have a new lake in the city. 536 00:38:43,320 --> 00:38:46,200 When you're confronted with footage as dramatic as this, 537 00:38:46,200 --> 00:38:48,520 you have to ask, what triggered it? 538 00:38:50,160 --> 00:38:51,760 Why did it happen now? 539 00:38:53,200 --> 00:38:57,360 The ancient geology of this state, the limestone, isn't enough. 540 00:39:02,720 --> 00:39:05,440 First glance, it must seem as if nowhere's safe in Florida, 541 00:39:05,440 --> 00:39:09,160 but actually some places are more at risk than others. 542 00:39:09,160 --> 00:39:13,280 This map shows the locations of all the verified sinkholes 543 00:39:13,280 --> 00:39:15,760 and you can see how widespread they are across the state. 544 00:39:15,760 --> 00:39:20,320 But there's a real cluster of them here in West Central Florida - 545 00:39:20,320 --> 00:39:23,040 a sinkhole sweet spot, if you like. 546 00:39:23,040 --> 00:39:26,560 And Jeff Bush's place is just in there, 547 00:39:26,560 --> 00:39:29,160 right on the southern edge of that sweet spot. 548 00:39:35,040 --> 00:39:36,520 On the face of it, 549 00:39:36,520 --> 00:39:39,720 this clustering of sinkholes doesn't make much sense. 550 00:39:41,760 --> 00:39:44,800 Why are some parts of the state relatively safe 551 00:39:44,800 --> 00:39:48,240 and others much more prone to sinkholes... 552 00:39:49,480 --> 00:39:54,040 ..even though they're all underlain by the same rock, limestone? 553 00:39:56,400 --> 00:39:59,080 So far, this search to understand what sinkholes 554 00:39:59,080 --> 00:40:02,960 are all about has focused on one material, rock. 555 00:40:02,960 --> 00:40:07,760 The thing is, voids in limestone open up ridiculously slowly, 556 00:40:07,760 --> 00:40:10,160 I mean, over thousands of years. 557 00:40:10,160 --> 00:40:12,400 So there's another material that we should consider, 558 00:40:12,400 --> 00:40:14,880 one that's far more mobile - 559 00:40:14,880 --> 00:40:16,080 soil. 560 00:40:25,840 --> 00:40:30,400 Geological mysteries seldom have just one culprit, 561 00:40:30,400 --> 00:40:34,840 and the material that covers this vast, unstable slab of limestone 562 00:40:34,840 --> 00:40:38,960 is rather more interesting than you might initially think. 563 00:40:38,960 --> 00:40:42,680 In fact, it determines how deadly sinkholes are. 564 00:40:43,920 --> 00:40:47,640 To understand why, you have to take a very close look 565 00:40:47,640 --> 00:40:49,400 at the soils around here. 566 00:40:52,240 --> 00:40:54,560 And the best way to do that is to take 567 00:40:54,560 --> 00:40:56,680 a trip down the Hillsborough River. 568 00:41:00,600 --> 00:41:04,120 The only problem with that is that the Hillsborough River 569 00:41:04,120 --> 00:41:06,240 is infested with alligators. 570 00:41:08,880 --> 00:41:11,160 I can't believe I'm risking my life for soil. 571 00:41:29,720 --> 00:41:31,640 There's one, there's one! 572 00:41:31,640 --> 00:41:34,360 There's one just there. Four, five-footer. 573 00:41:34,360 --> 00:41:36,080 Look at that. 574 00:41:36,080 --> 00:41:38,760 It's off, doesn't like me! 575 00:41:38,760 --> 00:41:40,920 I think it's more scared of me. 576 00:41:46,720 --> 00:41:49,520 The reason I'm out here on the Hillsborough River 577 00:41:49,520 --> 00:41:52,320 is that this area is almost sinkhole-free. 578 00:41:57,160 --> 00:42:00,560 But just downstream, it's a very different story. 579 00:42:03,560 --> 00:42:06,360 Look at this, a map of all the sinkholes in Florida. 580 00:42:06,360 --> 00:42:09,800 All these red squares here are crammed into West Central Florida. 581 00:42:09,800 --> 00:42:14,880 And then look, you get this kind of sinkhole-free area in here. 582 00:42:14,880 --> 00:42:17,720 To see that in more detail, we have to go to a different map. 583 00:42:17,720 --> 00:42:19,560 Look at this one here. 584 00:42:19,560 --> 00:42:21,000 This is the Hillsborough River, 585 00:42:21,000 --> 00:42:23,040 drifting in the Hillsborough River, 586 00:42:23,040 --> 00:42:24,640 just probably up here. 587 00:42:24,640 --> 00:42:26,640 We're just on that bend there. 588 00:42:26,640 --> 00:42:30,280 So, the thing is, in this area round here, there's just no sinkholes. 589 00:42:30,280 --> 00:42:33,600 But actually, to the west and to the east, it's sinkhole city. 590 00:42:33,600 --> 00:42:36,240 But to understand why there's no sinkholes here, 591 00:42:36,240 --> 00:42:38,280 we have to get to the shore. 592 00:42:50,440 --> 00:42:54,520 The limestone beneath me is the same as anywhere else in Florida 593 00:42:54,520 --> 00:42:57,560 and is riddled with caves and fissures. 594 00:42:57,560 --> 00:43:00,320 But it's the earth above that's different here. 595 00:43:01,440 --> 00:43:04,120 So what makes this place so safe? 596 00:43:06,080 --> 00:43:08,960 Time for a bit of old school geology. 597 00:43:13,000 --> 00:43:17,040 This is the instrument of a soil scientist, the auger. 598 00:43:17,040 --> 00:43:21,520 What you do is you clear a bit of ground 599 00:43:21,520 --> 00:43:23,120 and then you stick it in. 600 00:43:26,160 --> 00:43:29,440 Kind of just drill your way down through the vegetation. 601 00:43:33,120 --> 00:43:36,160 Can you hear the vegetation ripping? 602 00:43:36,160 --> 00:43:40,040 And then you get down into the soil, the topsoil, 603 00:43:40,040 --> 00:43:43,280 and as you get below that, 604 00:43:43,280 --> 00:43:45,800 it gets a little bit easier. In fact, it's easy now. 605 00:43:45,800 --> 00:43:48,960 So we can see what's underneath the topsoil. 606 00:43:48,960 --> 00:43:51,200 And what we see is, 607 00:43:51,200 --> 00:43:54,040 this is all soil but then this here, 608 00:43:54,040 --> 00:43:57,160 this is just sand. Look at that. 609 00:43:57,160 --> 00:44:00,920 Really coarse, loose sand. 610 00:44:00,920 --> 00:44:04,960 You can tell how coarse is it, cos when you chew it, 611 00:44:04,960 --> 00:44:07,240 it's quite gritty between your teeth. 612 00:44:07,240 --> 00:44:09,760 It's the sand that's key to there being 613 00:44:09,760 --> 00:44:12,720 so few sinkholes in this area. 614 00:44:12,720 --> 00:44:16,760 So the point is you've got this really loose, incohesive sand, 615 00:44:16,760 --> 00:44:18,640 and as the rainwater falls on it, 616 00:44:18,640 --> 00:44:21,040 to be honest, it just drains straight through. 617 00:44:21,040 --> 00:44:22,880 Straight through to the limestone below. 618 00:44:24,520 --> 00:44:26,920 This means that the soils never build up. 619 00:44:26,920 --> 00:44:29,960 The sands just fall into the voids as they form, 620 00:44:29,960 --> 00:44:34,040 so the closest thing you'll get to a big sinkhole round here 621 00:44:34,040 --> 00:44:36,760 is this rather innocuous puddle. 622 00:44:36,760 --> 00:44:39,200 The limestone underneath here would have started out 623 00:44:39,200 --> 00:44:42,360 something like that and then it's just been kind of dissolved down. 624 00:44:42,360 --> 00:44:45,640 And what you've had is you had the sand just kind of dripping, draining 625 00:44:45,640 --> 00:44:49,360 through it, and it just captures this, just a little patch of water. 626 00:44:49,360 --> 00:44:52,320 The limestone here is only a few feet beneath the 627 00:44:52,320 --> 00:44:57,000 thin sandy soil, which is constantly being washed down into it. 628 00:44:57,000 --> 00:45:00,680 Now, the thing is, these really aren't dangerous at all. 629 00:45:00,680 --> 00:45:03,680 There's hardly any chance of something like this collapsing. 630 00:45:05,920 --> 00:45:10,000 But a few miles down the river, you enter sinkhole alley. 631 00:45:11,000 --> 00:45:13,440 This is where the big sinkholes happen 632 00:45:13,440 --> 00:45:16,960 and the first thing you notice is the landscape's changed. 633 00:45:16,960 --> 00:45:20,560 We're out of the swamps and into an area rich in agriculture. 634 00:45:22,680 --> 00:45:26,240 During the winter, the fields around here are full of strawberries, 635 00:45:26,240 --> 00:45:30,400 making good use of the deep fertile soils and warm weather, 636 00:45:30,400 --> 00:45:33,040 so creating the fruit bowl of America. 637 00:45:37,400 --> 00:45:40,640 But I'm here in the off season and there's not a strawberry 638 00:45:40,640 --> 00:45:44,720 in sight, but that doesn't matter, as I'm only interested in the soil. 639 00:45:52,280 --> 00:45:55,680 You can see that's gone down much, much easier here. 640 00:45:55,680 --> 00:45:58,480 Now if I just pull that up now, let's see what we've got. 641 00:46:00,040 --> 00:46:04,240 OK, look, you can see it's much darker and it's still sandy, 642 00:46:04,240 --> 00:46:08,240 still pretty sandy but look, it's got some kind of strength to it, 643 00:46:08,240 --> 00:46:11,240 so that means there's some clay in there. 644 00:46:11,240 --> 00:46:14,000 Now if I went down another few metres or so, 645 00:46:14,000 --> 00:46:15,840 if I could be bothered going all the way down, 646 00:46:15,840 --> 00:46:18,440 I'd find something completely different. 647 00:46:18,440 --> 00:46:20,080 I'd find something like this. 648 00:46:20,080 --> 00:46:22,120 This is a clay and you can roll that. 649 00:46:22,120 --> 00:46:24,760 It's got some kind of strength. Look at that. 650 00:46:24,760 --> 00:46:26,200 Just not going to do anything. 651 00:46:26,200 --> 00:46:29,000 And it's that clay that turns out to be really important 652 00:46:29,000 --> 00:46:31,560 because that clay is really sticky. 653 00:46:31,560 --> 00:46:35,280 It's got its own strength and because of that, 654 00:46:35,280 --> 00:46:37,160 rather than as the water comes through, 655 00:46:37,160 --> 00:46:40,680 rather than it just washing the clay down into the sinkholes below, 656 00:46:40,680 --> 00:46:45,280 it stays there and it builds up, so you get a much thicker sequence 657 00:46:45,280 --> 00:46:48,600 of sediment and not just that, sediment with some strength to it. 658 00:46:50,040 --> 00:46:53,600 It's strong enough to bridge the holes in the limestone... 659 00:46:54,600 --> 00:46:56,000 ..but not for ever. 660 00:46:58,480 --> 00:47:00,880 Sooner or later, the clay will give way, 661 00:47:00,880 --> 00:47:04,560 and that's what happened at Winter Park 30 years ago. 662 00:47:05,720 --> 00:47:10,840 Back then, no-one knew there was a void beneath this part of town... 663 00:47:10,840 --> 00:47:15,920 but the whole place stood on a clay trap door, just waiting to spring. 664 00:47:25,880 --> 00:47:29,880 It's a strange idea that a layer of clay could be 665 00:47:29,880 --> 00:47:33,400 the cause for a potentially deadly sinkhole. 666 00:47:33,400 --> 00:47:36,360 Let me show you how, in a rather homespun way. 667 00:47:37,880 --> 00:47:39,640 What happens is this... 668 00:47:39,640 --> 00:47:41,920 Above the voids in the limestone, 669 00:47:41,920 --> 00:47:45,880 a layer of sand can form with a muddy clay above it 670 00:47:45,880 --> 00:47:48,240 and more soil on top of that. 671 00:47:52,480 --> 00:47:56,360 So you get this layer cake of sand, mud and sand. 672 00:47:58,480 --> 00:48:00,440 And then this is the tricky part. 673 00:48:02,240 --> 00:48:03,240 OK. 674 00:48:04,440 --> 00:48:07,520 And it creates a void in the sub-surface. Look at that. 675 00:48:07,520 --> 00:48:10,400 And it's the clay layer that's really important. 676 00:48:10,400 --> 00:48:13,840 It's forming a strong bridge that's holding the rest of that sand 677 00:48:13,840 --> 00:48:15,640 up and not letting it collapse. 678 00:48:16,680 --> 00:48:19,120 But if the clay loses its strength, 679 00:48:19,120 --> 00:48:23,200 the bridge it created fails...suddenly. 680 00:48:25,600 --> 00:48:27,040 The whole thing just caves in. 681 00:48:27,040 --> 00:48:29,480 It's kind of like a trap door that's been pulled 682 00:48:29,480 --> 00:48:32,200 and that is called a collapse sinkhole, 683 00:48:32,200 --> 00:48:34,120 and it's absolutely deadly. 684 00:48:39,920 --> 00:48:42,280 But what triggers that failure? 685 00:48:51,600 --> 00:48:54,440 This might seem like an urban paradise, 686 00:48:54,440 --> 00:48:57,280 but appearances can be deceptive. 687 00:48:59,320 --> 00:49:02,800 Many people here wonder if they're sitting on top of a fragile 688 00:49:02,800 --> 00:49:04,800 clay bridge into the underworld. 689 00:49:06,240 --> 00:49:10,680 So, alongside the cement mixers trying to stabilise the foundations, 690 00:49:10,680 --> 00:49:13,120 this is a pretty typical scene. 691 00:49:20,480 --> 00:49:22,480 Geologists drilling. 692 00:49:28,920 --> 00:49:32,200 What they're looking for is clay beneath the sands, 693 00:49:32,200 --> 00:49:36,000 a potential bridge which could fail at any moment. 694 00:49:40,960 --> 00:49:42,400 And if that clay were to fail, 695 00:49:42,400 --> 00:49:45,840 the cause will be something we're very familiar with. 696 00:49:50,720 --> 00:49:52,160 Water. 697 00:49:53,360 --> 00:49:57,120 The state of Florida is greedy for water 698 00:49:57,120 --> 00:50:01,200 to keep the lawns looking green, to fill swimming pools... 699 00:50:03,520 --> 00:50:08,200 ..and, most importantly of all, they need millions of gallons of water 700 00:50:08,200 --> 00:50:11,360 to grow the famous Florida strawberries and tomatoes. 701 00:50:13,080 --> 00:50:16,720 And the demand for all this water means the water table close 702 00:50:16,720 --> 00:50:20,400 to those fields can drop by over 50 feet in just a few days. 703 00:50:25,880 --> 00:50:30,480 And when the clay dries out, that lack of water can be the trigger. 704 00:50:35,360 --> 00:50:37,680 But that's just half the story. 705 00:50:39,400 --> 00:50:43,320 Water can also cause clay to fail because it becomes too wet. 706 00:50:45,760 --> 00:50:48,400 Florida famously is hurricane country. 707 00:50:52,600 --> 00:50:56,920 And when they hit, they can drop around a metre of water 708 00:50:56,920 --> 00:50:59,360 on the land in a matter of hours. 709 00:50:59,360 --> 00:51:01,120 Pretty bad out here now. 710 00:51:04,240 --> 00:51:07,320 The sheer wear of all that water on those fragile clay 711 00:51:07,320 --> 00:51:09,840 bridges can be enough to cause catastrophe... 712 00:51:12,520 --> 00:51:15,000 ..so, ultimately, water, 713 00:51:15,000 --> 00:51:19,240 too much or too little, can be the trigger for a sinkhole collapse. 714 00:51:30,880 --> 00:51:33,160 But if water can be the trigger, 715 00:51:33,160 --> 00:51:36,840 then there's another reason why Florida's so prone to sinkholes. 716 00:51:40,520 --> 00:51:45,880 And when you've spent a bit of time here, it starts to become obvious. 717 00:51:45,880 --> 00:51:47,400 It's something to do with us. 718 00:51:48,560 --> 00:51:51,160 You know, the sinkholes, sinkholes have always been here. 719 00:51:51,160 --> 00:51:55,440 They're part of Florida's constantly evolving geology. 720 00:51:55,440 --> 00:51:57,640 But what's changed is us. 721 00:51:57,640 --> 00:52:00,920 I mean, our fondness for the sun, our expanding numbers, 722 00:52:00,920 --> 00:52:03,160 we're taking over more and more of the land. 723 00:52:11,280 --> 00:52:16,040 As the population of Florida grows, people need somewhere to live 724 00:52:16,040 --> 00:52:19,640 and that means we inevitably end up 725 00:52:19,640 --> 00:52:22,480 building on more sinkhole-prone land. 726 00:52:26,480 --> 00:52:29,760 It's easy to get the impression here that this is a state 727 00:52:29,760 --> 00:52:31,280 collapsing in on itself. 728 00:52:39,200 --> 00:52:44,640 There are almost 6,700 sinkhole-related claims each year, 729 00:52:44,640 --> 00:52:46,200 and the numbers are rising. 730 00:52:48,560 --> 00:52:51,960 But incredibly, they've only killed three people in the last 731 00:52:51,960 --> 00:52:55,480 40 years as few collapse without warning. 732 00:52:57,600 --> 00:53:01,080 What's far more typical is what happened at this restaurant. 733 00:53:01,080 --> 00:53:04,360 Just a few weeks back, the staff came in in the morning to find 734 00:53:04,360 --> 00:53:08,760 that cracks had appeared overnight, so they called in the engineers 735 00:53:08,760 --> 00:53:12,440 and basically they - look at it - condemned the whole building. 736 00:53:14,560 --> 00:53:17,080 30-odd people used to work here. 737 00:53:17,080 --> 00:53:20,960 But the thing is, because of those warning signs, there was no tragedy. 738 00:53:27,120 --> 00:53:30,880 Sinkholes may seem as old as the Earth itself, 739 00:53:30,880 --> 00:53:34,680 but for millions of years those deep voids have been forming 740 00:53:34,680 --> 00:53:38,440 incredibly slowly, collapsing down only rarely. 741 00:53:40,880 --> 00:53:46,360 In a sense, sinkhole-related damage comes from what we're doing... 742 00:53:46,360 --> 00:53:51,240 how we use our resources like water, and especially the fact that 743 00:53:51,240 --> 00:53:55,320 we're building on deep clay soils above voids like this. 744 00:54:06,160 --> 00:54:10,800 What happened to Jeff on the 28th of February, 2013, 745 00:54:10,800 --> 00:54:13,560 when a sinkhole opened up under his bedroom, 746 00:54:13,560 --> 00:54:17,040 sucking him deep into the earth, is mercifully rare. 747 00:54:17,040 --> 00:54:20,880 The bedroom floor just collapsed and my brother-in-law is in there. 748 00:54:20,880 --> 00:54:23,120 He's underneath the house. 749 00:54:23,120 --> 00:54:25,360 It was a freak combination of factors... 750 00:54:27,840 --> 00:54:31,080 ..shaped by the geology of where he lived, certainly, 751 00:54:31,080 --> 00:54:33,880 but also due to tragic bad luck. 752 00:54:36,720 --> 00:54:40,480 What's clear is that Jeff's house lay in sinkhole alley. 753 00:54:42,480 --> 00:54:45,600 With its clay soil, it's prone to sudden sinkholes. 754 00:54:49,320 --> 00:54:52,720 A structural engineer who worked with the emergency workers 755 00:54:52,720 --> 00:54:54,600 that night was Bill Bracken. 756 00:54:56,640 --> 00:54:59,360 The sinkhole was centred exactly on the bedroom, 757 00:54:59,360 --> 00:55:02,960 and was contained completely within the footprint of the building, 758 00:55:02,960 --> 00:55:05,760 so it was not visible from the air, from outside. 759 00:55:05,760 --> 00:55:08,640 The only way to see that hole was to look in the window or 760 00:55:08,640 --> 00:55:10,040 be inside the structure. 761 00:55:16,360 --> 00:55:18,400 When they poked a remote camera through, 762 00:55:18,400 --> 00:55:22,440 they saw a neat hole in the concrete slab of Jeff's bedroom 763 00:55:22,440 --> 00:55:25,520 and a near vertical shaft over 20 feet deep. 764 00:55:30,160 --> 00:55:34,720 The sinkhole formed and the soil began to drop. 765 00:55:34,720 --> 00:55:37,640 Because you had the entire structure over the top of this, 766 00:55:37,640 --> 00:55:40,720 because it was completely contained inside the footprint 767 00:55:40,720 --> 00:55:44,160 of the building, when those soils began to drop, 768 00:55:44,160 --> 00:55:49,560 it was almost a flushing action that created a suction force, 769 00:55:49,560 --> 00:55:54,520 so it wasn't as though a hole opened up and things fell into it. 770 00:55:54,520 --> 00:55:57,160 It was almost as though, as the soil began to pull 771 00:55:57,160 --> 00:55:59,600 away from the underside and a void was being created, 772 00:55:59,600 --> 00:56:01,680 a suction force was being created as well. 773 00:56:01,680 --> 00:56:04,680 Just tugged at the bottom of the concrete base of the bedroom? 774 00:56:04,680 --> 00:56:07,720 So, it effectively pulled that slab down into the hole. 775 00:56:09,440 --> 00:56:13,040 When fire crews placed a listening device at the bottom, 776 00:56:13,040 --> 00:56:15,240 they were in for a shock - 777 00:56:15,240 --> 00:56:17,080 the sinkhole was still alive. 778 00:56:19,480 --> 00:56:22,360 By the time that they had deployed that, set that in there, 779 00:56:22,360 --> 00:56:25,000 placed that on, on the ground surface, 780 00:56:25,000 --> 00:56:29,480 walked back to the box, connected it and began receiving signals, 781 00:56:29,480 --> 00:56:33,720 they began to sense a tugging, if you will, on the cable. 782 00:56:33,720 --> 00:56:35,920 When they went to check, 783 00:56:35,920 --> 00:56:39,400 the listening device had already been pulled down into the ground, 784 00:56:39,400 --> 00:56:42,640 so over the next three days there was about 30 feet of cable that 785 00:56:42,640 --> 00:56:46,200 was draped out of the window and on the ground, and we watched that 786 00:56:46,200 --> 00:56:50,400 30 feet of cable reduce down to about six-to-eight feet of cable. 787 00:56:50,400 --> 00:56:53,120 So we knew that that sand was still in an excited state. 788 00:56:53,120 --> 00:56:54,880 It was still pulling down. 789 00:56:54,880 --> 00:56:58,200 Everything that landed on it was immediately pulled down in, 790 00:56:58,200 --> 00:57:03,360 so we knew that whatever had gone into that was down at least 791 00:57:03,360 --> 00:57:05,000 30, 35 feet. 792 00:57:05,000 --> 00:57:08,440 SIREN BLARES 793 00:57:12,120 --> 00:57:15,320 The one certainty is that when the emergency services 794 00:57:15,320 --> 00:57:19,440 sent down a probe to look for signs of life, they found none. 795 00:57:25,920 --> 00:57:29,480 All that's left now is a family trying to come to terms with 796 00:57:29,480 --> 00:57:32,200 the freak tragic events of that night. 797 00:57:34,960 --> 00:57:40,720 They told us that they were not for sure how far down he was 798 00:57:40,720 --> 00:57:45,000 and that the ground around everything was just too... 799 00:57:46,440 --> 00:57:50,920 ..too unsafe, too unstable for them to do anything. 800 00:57:50,920 --> 00:57:52,920 They could not risk anybody else's life. 801 00:58:16,360 --> 00:58:19,760 It makes me sick, just the way it looks. 802 00:58:19,760 --> 00:58:24,640 Overgrown with grass, nobody taking care of anything. And there's a... 803 00:58:25,840 --> 00:58:27,560 My brother's down there still.