0:00:09 > 0:00:13Seffner, Florida.
0:00:13 > 0:00:15A place where the earth opened up...
0:00:16 > 0:00:19..and killed a man.
0:00:19 > 0:00:21Lazy Lanes, this place is called.
0:00:21 > 0:00:24It just seems so ordinary, so normal.
0:00:24 > 0:00:25DOG BARKS
0:00:25 > 0:00:28I mean, look at this, "Beware of the dog."
0:00:28 > 0:00:35And yet this whole estate sits above a trap door into the hidden Florida.
0:00:35 > 0:00:38'The bedroom floor just collapsed
0:00:38 > 0:00:41'and my brother-in-law is underneath the house.'
0:00:41 > 0:00:44The trap door into that hidden Florida opened here,
0:00:44 > 0:00:47in February 2013...
0:00:49 > 0:00:52..creating a sinkhole.
0:00:52 > 0:00:56Without warning, it swallowed everything in this bedroom,
0:00:56 > 0:01:01and for those who saw it, it was something they'll never forget.
0:01:01 > 0:01:04It's like this thing was alive.
0:01:04 > 0:01:07You know, it was turning, moving around,
0:01:07 > 0:01:10making noises, you know, almost like a growl.
0:01:16 > 0:01:18Sinkholes don't just happen in Florida.
0:01:19 > 0:01:22They're occurring all over the world.
0:01:25 > 0:01:28I want to find out why sinkholes form...
0:01:30 > 0:01:33..what this underworld is really like.
0:01:33 > 0:01:35You've not been down there?
0:01:35 > 0:01:37Never been in there, didn't even know it existed.
0:01:37 > 0:01:41And why some are deadly.
0:02:06 > 0:02:09Until last spring, a house stood here.
0:02:10 > 0:02:13A home lived in by two brothers,
0:02:13 > 0:02:16Jeff and Jeremy Bush, along with their families.
0:02:18 > 0:02:20My bedroom was right here.
0:02:20 > 0:02:21So, your bedroom was here?
0:02:21 > 0:02:24Yeah, my bedroom was right here in the front.
0:02:24 > 0:02:27And you walked in through the front door, and there was a living room.
0:02:27 > 0:02:29Yeah, so, you went in and then the living room was on the right?
0:02:29 > 0:02:32Yeah, living room was on the right, kitchen behind my bedroom,
0:02:32 > 0:02:35and then the dining room and then my brother's room.
0:02:35 > 0:02:38That was... So it was on the far right-hand side.
0:02:38 > 0:02:41- And then there was another bedroom, it was Janell's...- Oh, right.
0:02:41 > 0:02:42It was Janell's bedroom.
0:02:43 > 0:02:45It was a normal night, I guess?
0:02:45 > 0:02:46It was, it was normal.
0:02:46 > 0:02:51I had just got home from work,
0:02:51 > 0:02:55come in about ten, 10.30,
0:02:55 > 0:02:58and told everybody good night and went to bed.
0:02:58 > 0:03:01And then that's when it happened.
0:03:02 > 0:03:05My wife turned the light on, and...
0:03:07 > 0:03:09..as I was getting ready to walk in the door,
0:03:09 > 0:03:14she turned the light on and all you could see was this big hole.
0:03:14 > 0:03:18It sounded like a car hit the house,
0:03:18 > 0:03:21but the house didn't move, um, nothing moved.
0:03:21 > 0:03:24The walls didn't move, nothing.
0:03:24 > 0:03:27Pictures were still hanging on the wall, everything.
0:03:27 > 0:03:30It just was a loud crash.
0:03:34 > 0:03:37That's a scream I'll never forget.
0:03:37 > 0:03:38Um...
0:03:42 > 0:03:45He just kept saying, "Help me, somebody help me."
0:03:45 > 0:03:49SIREN WAILS
0:03:49 > 0:03:53That scream was the last anyone heard of Jeff Bush.
0:03:53 > 0:03:55No-one had any idea what was happening,
0:03:55 > 0:03:58just that he was in trouble.
0:03:58 > 0:03:59It was a 911 call
0:03:59 > 0:04:04and it said that a family member had fallen underneath the house.
0:04:17 > 0:04:21The first one there was Deputy Duvall.
0:04:21 > 0:04:22I guess my first idea
0:04:22 > 0:04:25was it may have been, like, an accident-type,
0:04:25 > 0:04:27like, somebody was trying to renovate their house
0:04:27 > 0:04:29and something happened with the floor,
0:04:29 > 0:04:32it was rotten, you know, something of that sort.
0:04:32 > 0:04:34He rushed straight in.
0:04:34 > 0:04:36He went straight in the house,
0:04:36 > 0:04:39and he pulled a couple of people out of the house,
0:04:39 > 0:04:40made them get out of the house.
0:04:40 > 0:04:44Everybody was screaming and kind of running around.
0:04:44 > 0:04:48As soon as I saw them, I knew that it wasn't just somebody
0:04:48 > 0:04:52that had fallen into the floor by an accident, you know?
0:04:52 > 0:04:54I knew just from their reactions
0:04:54 > 0:04:56that it was something a lot more significant.
0:04:56 > 0:05:00I looked inside the room. There was nothing.
0:05:00 > 0:05:02All you could do was smell fresh dirt.
0:05:02 > 0:05:04It took his whole bedroom.
0:05:04 > 0:05:08The only piece of concrete that was left was by the door.
0:05:08 > 0:05:11It took his bed, his dresser, his TV and everything down in the hole.
0:05:11 > 0:05:14The door was open and when I went through the house,
0:05:14 > 0:05:16everything looked like it was normal.
0:05:16 > 0:05:19You know, the floor was intact and the lights were on,
0:05:19 > 0:05:20the power was working.
0:05:20 > 0:05:23Nothing out of the ordinary - but I went to the bedroom,
0:05:23 > 0:05:27and the door was open, and as soon as I turned and looked in,
0:05:27 > 0:05:30there was nothing in the bedroom. It was just a giant hole.
0:05:32 > 0:05:36Jeff had been pulled down into the underworld by a sinkhole.
0:05:39 > 0:05:42All I could see was the tip of his box spring,
0:05:42 > 0:05:46the tip of his bed frame and his mattress, and that was it.
0:05:46 > 0:05:47You didn't see Jeff at that point?
0:05:47 > 0:05:50Couldn't see Jeff, I thought I... I thought I could hear him
0:05:50 > 0:05:53- yelling for me to help him. - Yeah, and so you just...
0:05:53 > 0:05:56I just jumped in there and started digging.
0:05:56 > 0:05:59You want to take a minute?
0:05:59 > 0:06:03Jeremy was... If you're walking into the actual bedroom,
0:06:03 > 0:06:07he's pretty much right there between the door and the centre of the hole.
0:06:07 > 0:06:09I started digging by my hands,
0:06:09 > 0:06:12and I was yelling and screaming for him,
0:06:12 > 0:06:14and yelling for my father-in-law to get a shovel
0:06:14 > 0:06:17and a flashlight, so I could see.
0:06:17 > 0:06:21So I grabbed hold of his bed and tried moving his bed,
0:06:21 > 0:06:22and it wouldn't move
0:06:22 > 0:06:26and I broke the bedframe in half trying to get it out.
0:06:26 > 0:06:29Then I started... He got me the shovel and I started digging.
0:06:29 > 0:06:33And they were real close. You've seen Jeremy, you've seen Jeff.
0:06:33 > 0:06:36And they worked together, they did everything together.
0:06:36 > 0:06:39They played video games together, they...
0:06:40 > 0:06:43They were... They were brothers, they were tight brothers.
0:06:46 > 0:06:49Jeff Bush had been swallowed alive,
0:06:49 > 0:06:53and the sinkhole came close to swallowing his brother, Jeremy, too.
0:06:55 > 0:06:58The ground was still falling as I was in the hole
0:06:58 > 0:07:02and the concrete was moving and breaking, still.
0:07:02 > 0:07:04- So, you were on your way down? - I was on my way down.
0:07:04 > 0:07:07- And you didn't even notice it?- I wasn't paying attention to it at all.
0:07:07 > 0:07:09I was just trying to get my brother out.
0:07:09 > 0:07:13If you can imagine an hourglass, the funnel inside of an hourglass,
0:07:13 > 0:07:16you've got the deeper portion,
0:07:16 > 0:07:18then you've got the out... the outermost wall.
0:07:18 > 0:07:22Jeremy was in the middle and while it was sinking,
0:07:22 > 0:07:23it was also expanding out
0:07:23 > 0:07:28because everything on the outside was filling in the void.
0:07:28 > 0:07:30I know I didn't want to come out the hole,
0:07:30 > 0:07:34I wanted to pull the deputy in there with me to help me dig him out,
0:07:34 > 0:07:36- because nobody was helping. - Of course.
0:07:40 > 0:07:44Those who saw it forming will never forget it.
0:07:44 > 0:07:46When everybody asks me about it,
0:07:46 > 0:07:48I tell them it's like this thing was alive,
0:07:48 > 0:07:52and when I say that it was eating, it literally...
0:07:53 > 0:07:56Things, the furniture, it was still sinking,
0:07:56 > 0:07:58it was still going into the ground.
0:07:58 > 0:08:03And, you know, it was kind of, kind of turning, moving around,
0:08:03 > 0:08:05making noises, you know,
0:08:05 > 0:08:09almost like a growl, and it just... Like something was alive.
0:08:15 > 0:08:17Jeremy was seconds from being sucked down
0:08:17 > 0:08:20to the same terrible fate as his brother
0:08:20 > 0:08:22when Deputy Duvall saved him.
0:08:23 > 0:08:26He grabbed me by my arm and snatched me out of the hole,
0:08:26 > 0:08:30and that was the last time that anybody went back in the house.
0:08:41 > 0:08:45When the sun came up, everything seemed normal from the outside.
0:08:45 > 0:08:48All anyone knew was that Jeff Bush was gone,
0:08:48 > 0:08:50his body never to be discovered.
0:08:55 > 0:08:58I found meeting Jeremy and hearing what happened to his brother
0:08:58 > 0:09:01deeply disturbing and unsettling.
0:09:03 > 0:09:05Such a horrible thought, isn't it?
0:09:05 > 0:09:10That idea of the ground opening up and literally swallowing you alive,
0:09:10 > 0:09:13it's just... It's the stuff of nightmares.
0:09:18 > 0:09:22My science, geology, tries to give answers to why things happen
0:09:22 > 0:09:24and hopefully save lives.
0:09:26 > 0:09:30And one of the reasons I've become so interested in sinkholes right now
0:09:30 > 0:09:32is that I've noticed more of them in the news.
0:09:39 > 0:09:41In the last few years,
0:09:41 > 0:09:44they have been captured as far afield as China...
0:09:47 > 0:09:48..and Guatemala.
0:09:50 > 0:09:53They create the fear that the ground beneath our feet
0:09:53 > 0:09:56could open up into an unseen world at any time.
0:10:00 > 0:10:03Incredibly, this young girl survived.
0:10:07 > 0:10:09But it's in Florida
0:10:09 > 0:10:12where the fear of dropping into the underworld is greatest.
0:10:16 > 0:10:18A few months after Jeff Bush died,
0:10:18 > 0:10:21over 100 people were saved from this resort complex
0:10:21 > 0:10:24near Disney World just before it collapsed.
0:10:30 > 0:10:34The dubious honour of being called the sinkhole capital of the world
0:10:34 > 0:10:38costs the state of Florida hundreds of millions of dollars a year.
0:10:39 > 0:10:41So at least lawyers are getting something out of it.
0:10:41 > 0:10:43Look at that.
0:10:43 > 0:10:45As soon as homeowners see some kind of crack in their house,
0:10:45 > 0:10:48they're encouraged to phone these sinkhole attorneys.
0:10:50 > 0:10:51And they ply their trade
0:10:51 > 0:10:54up and down the highway here with all their billboards.
0:11:03 > 0:11:07I've never seen a sinkhole before, not up close,
0:11:07 > 0:11:11but to really understand them you can't just look at the surface.
0:11:11 > 0:11:14You have to see what's happening underneath.
0:11:14 > 0:11:19If you really want to know what the Florida underworld is like, you've got to descend into it.
0:11:19 > 0:11:20On the rope!
0:11:29 > 0:11:32There's one thing I could tell straightaway -
0:11:32 > 0:11:35the rock I'm descending through is very familiar.
0:11:37 > 0:11:40It's one of the most common rock types in the world
0:11:40 > 0:11:44and obviously one of the most useful - it's a raw ingredient for cement.
0:11:44 > 0:11:48But it's also the maker of these fantastic subterranean worlds.
0:11:54 > 0:11:57If you were walking above, you'd never know this was here.
0:11:59 > 0:12:03A great void, taking shape beneath the surface of the earth.
0:12:06 > 0:12:11Professor Jason Polk has been trying to understand what it is that makes them collapse.
0:12:13 > 0:12:19He estimates that the caves started forming millions of years ago.
0:12:19 > 0:12:22That's the last kind of part of it, breaching to the surface?
0:12:22 > 0:12:25Eventually the rock becomes so thin
0:12:25 > 0:12:28that a large collapse can occur instantaneously.
0:12:28 > 0:12:31And a lot of the sinkholes you see in Florida where
0:12:31 > 0:12:35you have those instant collapses are where it's thin rock and thick soil.
0:12:35 > 0:12:39So what's your best bet about how old that collapse was, when did daylight come in here?
0:12:39 > 0:12:42Best guess is from work we've done with the sediments where
0:12:42 > 0:12:46we've done radiocarbon dating to actually see how old these sediments are,
0:12:46 > 0:12:48and we know that they are at least 10,000 years old,
0:12:48 > 0:12:53- so that collapse is probably 10,000 years old at a minimum.- Yeah.
0:12:59 > 0:13:02It may look like the sinkhole's dead, but it's anything but.
0:13:05 > 0:13:10Ever since the roof fell in, the cave has been filling with sediment.
0:13:10 > 0:13:15But then three years ago, that sediment disappeared.
0:13:15 > 0:13:18So 2010 then, it was up to there, is that right?
0:13:18 > 0:13:20You would have been under sediment.
0:13:20 > 0:13:23And just within minutes, hours - straight down?
0:13:23 > 0:13:26- Just almost immediately. - And where, down...down here?
0:13:26 > 0:13:29Straight down here in this hole, which is the pathway to some
0:13:29 > 0:13:33unknown void below where all of this sediment is washed down and continues to wash down.
0:13:35 > 0:13:39- You've not been down there?- Never been in there, didn't even know it existed.- God!
0:13:39 > 0:13:42So do you reckon it's...? I mean, are you slightly freaked out
0:13:42 > 0:13:44that underneath we know there's a big hole?
0:13:44 > 0:13:48It feels solid but we know that if we'd have been here a few years ago when this happened, we'd be...
0:13:48 > 0:13:51- We'd be down there. - We'd be down below in the unknown.
0:13:51 > 0:13:54- Wow, that's a bit scary. - We should go.
0:13:54 > 0:13:55I don't like that, actually.
0:13:55 > 0:14:00That gaping hole showed how alive this sinkhole still is.
0:14:00 > 0:14:02I was very glad to leave.
0:14:14 > 0:14:17I've been told that if I want to get a sense of just what
0:14:17 > 0:14:21a problem sinkholes really are here, there's somewhere I should go.
0:14:23 > 0:14:25This estate is just north of Tampa.
0:14:27 > 0:14:30In many ways, it's a pretty unremarkable place.
0:14:32 > 0:14:36Full of retirees chasing the dream of all year round sun.
0:14:36 > 0:14:39It all looks so perfect, doesn't it? I mean, look at it.
0:14:39 > 0:14:42But behind the facade, virtually all of these houses have got
0:14:42 > 0:14:45structural problems and cracks and got people going to
0:14:45 > 0:14:48bed at night really not sure if those sounds that they're
0:14:48 > 0:14:52hearing is going to be another hole appearing underneath them.
0:14:57 > 0:15:00So instead of spending their time making extensions and patios...
0:15:04 > 0:15:08..the builders round here are pouring hundreds of tonnes of cement beneath
0:15:08 > 0:15:11the foundations of these houses in an attempt to stabilise them.
0:15:20 > 0:15:24'I went to visit one of the long-standing residents, Darlene Denaro.'
0:15:26 > 0:15:29So how many of these houses would you say have had problems
0:15:29 > 0:15:32or are getting problems now?
0:15:32 > 0:15:37Erm...well, she has one, and in one down there on that side...
0:15:37 > 0:15:39- By the white car? - By the white car,
0:15:39 > 0:15:44- and then across the street there's one that was fixed.- Right.
0:15:44 > 0:15:47Yeah, then this one and this one,
0:15:47 > 0:15:51and then she suspects she has a sinkhole.
0:15:51 > 0:15:54Which one, which one, the one with the green car?
0:15:54 > 0:15:57- No, on the other side. - Other side of that?
0:15:57 > 0:16:00Yeah, and then around the corner, where Louis lives,
0:16:00 > 0:16:04he lives on this side right here and he had, they had...
0:16:04 > 0:16:10It's one, two, three and then one down here, he had a humungous one.
0:16:10 > 0:16:14I think he had pretty close to 70 truck loads.
0:16:14 > 0:16:17So you're kind of in the centre, really, of this... Sorry, well,
0:16:17 > 0:16:19yeah, I was thinking, I mean, that...
0:16:19 > 0:16:23I feel like I'm in... Yeah, I'm in the centre.
0:16:27 > 0:16:31For decades, new housing has been springing up all across Florida.
0:16:33 > 0:16:35Built on what was once rough farmland,
0:16:35 > 0:16:39they seemed to offer a golden future for their new inhabitants.
0:16:41 > 0:16:43But now I get the sense that many of them
0:16:43 > 0:16:48are gripped by a collective fear of what might lie beneath.
0:16:49 > 0:16:53- When you moved here, was anything mentioned about sinkholes here?- Nothing.
0:16:53 > 0:16:57- What was it before? - Farmland, wet farmland, swampland.
0:16:57 > 0:17:00Everybody who's... The old Florida people that had been
0:17:00 > 0:17:05here for years, it was a very wet, soggy, swamp piece of property.
0:17:05 > 0:17:07We would have never moved here, never.
0:17:07 > 0:17:08- If you'd known.- No.
0:17:18 > 0:17:22What's happening on this estate is not that unusual -
0:17:22 > 0:17:25sinkholes ruining the American Dream.
0:17:27 > 0:17:29Look at this. Here we've got one.
0:17:30 > 0:17:33For a lot of people, this is reality now,
0:17:33 > 0:17:36trying to fill in this huge void underneath their houses.
0:17:40 > 0:17:43I detected a real fear on this estate.
0:17:43 > 0:17:48After all, Jeff Bush lived just an hour down the road.
0:17:48 > 0:17:50But how justified are those fears?
0:18:01 > 0:18:06It's very hard to get a sense of how many sinkholes there are in this state.
0:18:13 > 0:18:15And just what a threat they are to the people living here.
0:18:23 > 0:18:26So I'm going to try and get a rather higher perspective on the problem.
0:18:26 > 0:18:28RADIO CHATTER
0:18:39 > 0:18:42It's going to be so good to get up top because, I mean, to be
0:18:42 > 0:18:45honest, on the ground, it's quite a tedious landscape.
0:18:45 > 0:18:49It's very flat, there's lots of trees and tarmac that obscure
0:18:49 > 0:18:53the views, but I think that from here, everything will become clear.
0:19:01 > 0:19:04So we're going to go up about 500 feet, is that right?
0:19:05 > 0:19:07- Above sea level.- Right.
0:19:10 > 0:19:14What I'm looking for is evidence of ancient sinkholes,
0:19:14 > 0:19:17depressions in the land that have formed into lakes.
0:19:17 > 0:19:20You can start to see some now. You see that over there?
0:19:29 > 0:19:32Just a pockmark, a series of little lakes.
0:19:33 > 0:19:37Like the whole place now is just lake land, everywhere you can see.
0:19:37 > 0:19:39It's just little pockets, like just here.
0:19:44 > 0:19:47Looks like the whole place is a giant golf course with kind of water hazards.
0:19:47 > 0:19:50This is pretty much Florida, I guess.
0:19:52 > 0:19:54Each one of those lakes beneath us
0:19:54 > 0:19:57started as a hole into a limestone cavern beneath.
0:20:06 > 0:20:09It's quite interesting, there's this intricate anatomy to them.
0:20:09 > 0:20:11Some of them are just, just perfect circles,
0:20:11 > 0:20:14and that's just one sinkhole, but others you can see,
0:20:14 > 0:20:16it's like four or five have all joined together.
0:20:19 > 0:20:22So the whole thing is just pockmarked as far as you can see,
0:20:22 > 0:20:25filled with water, so you get these ornamental water features
0:20:25 > 0:20:28that people build their houses and jetties around.
0:20:28 > 0:20:30They're really sought-after.
0:20:35 > 0:20:39But the thing is, potentially, they're lethal. I mean, these things can open up
0:20:39 > 0:20:41and you're left with a hole in the ground.
0:20:43 > 0:20:47It's reckoned there are several thousand of these sinkhole
0:20:47 > 0:20:51lakes across the state of Florida, several thousand of them!
0:20:51 > 0:20:54And that's just a part of it.
0:20:54 > 0:20:58I tried to get a figure on just how many sinkholes there are in the state.
0:20:58 > 0:21:01One county claims to have over 6,000 of them,
0:21:01 > 0:21:04but the truth is, no-one really knows.
0:21:04 > 0:21:06They've literally stopped counting.
0:21:25 > 0:21:27To understand why,
0:21:27 > 0:21:32you have to travel back to the very origins of the state itself.
0:21:33 > 0:21:38Head a couple of miles out to sea and you can observe the whole process beginning.
0:21:46 > 0:21:48Tens of millions of years ago,
0:21:48 > 0:21:53modern Florida emerged as shallow seas just like this receded.
0:21:56 > 0:21:59Warm seas that once teemed with marine life.
0:22:13 > 0:22:16Quite nice, nice temperature.
0:22:16 > 0:22:18No need for a snorkel and flippers.
0:22:18 > 0:22:23It's amazingly shallow, even two miles off the shore.
0:22:23 > 0:22:26For a wee walk in the Florida Keys.
0:22:29 > 0:22:32What I'm standing on is the remains of the marine life that
0:22:32 > 0:22:36inhabited this warm shallow coastline.
0:22:36 > 0:22:38And the remains of those creatures
0:22:38 > 0:22:41form a mud made from carbonates.
0:22:42 > 0:22:44When those creatures die and decompose,
0:22:44 > 0:22:47they turn into this, carbonate mud.
0:22:47 > 0:22:50It's amazing to think this stuff is just the smashed-up
0:22:50 > 0:22:53hard parts of millions of sea creatures.
0:22:53 > 0:22:56And yet this is Florida in the making.
0:22:56 > 0:22:59Over hundreds of thousands of years it gets compressed
0:22:59 > 0:23:03into limestone, the rock that virtually all of Florida is made of.
0:23:10 > 0:23:15An entire peninsula, around 500 miles north to south
0:23:15 > 0:23:18and over 160 east to west.
0:23:18 > 0:23:22And almost every inch of that is made of marine organisms.
0:23:22 > 0:23:25And that's the key to sinkholes, really,
0:23:25 > 0:23:28because what can be created can also be destroyed.
0:23:46 > 0:23:50This is the bedrock of Florida stripped bare -
0:23:50 > 0:23:52an old quarry at Windley Key.
0:24:01 > 0:24:04If this looks solid and unchanging, it isn't.
0:24:04 > 0:24:06CREAKING
0:24:08 > 0:24:12Something has been eating it away and still is.
0:24:19 > 0:24:22The thing is, everywhere you look, this rock is being destroyed.
0:24:22 > 0:24:24I mean, look at this bit here.
0:24:24 > 0:24:27It's been eroded away.
0:24:27 > 0:24:29These are kind of miniature sinkholes, really.
0:24:33 > 0:24:36And what's created them is just rain.
0:24:36 > 0:24:39The rain is falling down and dissolving them away.
0:24:41 > 0:24:45But to eat away the limestone, that rain has to change.
0:24:45 > 0:24:49Pure water has a pH of around about seven,
0:24:49 > 0:24:51but as it falls through the atmosphere it picks up
0:24:51 > 0:24:55carbon dioxide molecules that turns it into a very weak acid,
0:24:55 > 0:24:56carbonic acid.
0:24:56 > 0:24:59And the other thing is that if it hits, here,
0:24:59 > 0:25:00rotting vegetation and soil,
0:25:00 > 0:25:03then that pH drops even more.
0:25:03 > 0:25:07So, look at this. This is a pH meter and it's reading seven.
0:25:07 > 0:25:10Now, if I stick it into this soil here,
0:25:10 > 0:25:13it's gone down - 6.6, 5.3.
0:25:13 > 0:25:16So that is really quite dramatically more acidic.
0:25:16 > 0:25:20It's that acidity over thousands, tens of thousands of years,
0:25:20 > 0:25:23that basically eats out those huge caves.
0:25:23 > 0:25:26And if you don't believe me that acid can dissolve away rock,
0:25:26 > 0:25:28I've got a little bit of acid.
0:25:28 > 0:25:30It's hydrochloric acid, but it's quite dilute.
0:25:30 > 0:25:33Look, if I pour it on my skin, it does nothing.
0:25:35 > 0:25:38But if I pour it on this fossil coral...
0:25:38 > 0:25:40look at that.
0:25:40 > 0:25:42It's just going crazy.
0:25:44 > 0:25:46One moment you've got calcium carbonate,
0:25:46 > 0:25:50next moment it's all fizzed back to carbon dioxide.
0:25:50 > 0:25:54And it's this process that's very slowly dissolving the whole state.
0:25:58 > 0:25:59One of the things that drew me
0:25:59 > 0:26:02to geology is how it makes you see the certainties
0:26:02 > 0:26:07of the world we've created, the human planet, rather differently.
0:26:07 > 0:26:11There are almost 20 million people living in Florida
0:26:11 > 0:26:13and the population's growing rapidly.
0:26:14 > 0:26:17Driving along the streets of somewhere like Miami, you feel
0:26:17 > 0:26:21as if you're in one of the safest, most modern places in the world.
0:26:25 > 0:26:28It's all built on rocks that are being dissolved by water.
0:26:28 > 0:26:32Every drop that falls from the sky,
0:26:32 > 0:26:37every drop that sinks through the ground, is turning to acid,
0:26:37 > 0:26:42and that acid is very slowly dissolving the whole state.
0:26:46 > 0:26:50To witness it in action, you have to leave the urban sprawl
0:26:50 > 0:26:52and head out into the old Florida,
0:26:52 > 0:26:56the Florida that existed before people ever set foot here.
0:26:59 > 0:27:01Step just a few feet from the freeway
0:27:01 > 0:27:04and you're into this primeval land of swamp and alligator.
0:27:07 > 0:27:11It's a world I'm entering in search of something rather special.
0:27:29 > 0:27:32I'm off to meet up with a remarkable team of explorers
0:27:32 > 0:27:34who will really be able to show me
0:27:34 > 0:27:36why there are so many sinkholes in Florida.
0:27:42 > 0:27:44This is Peacock Springs.
0:27:47 > 0:27:49It's an alien place.
0:27:52 > 0:27:55It's a part of Florida that most tourists,
0:27:55 > 0:27:58even most residents, never see.
0:28:02 > 0:28:03It's like a lost world.
0:28:16 > 0:28:20The water is just so clear.
0:28:20 > 0:28:22And so it should be, really,
0:28:22 > 0:28:26because this is Florida's lifeblood.
0:28:26 > 0:28:29Something like 95% of the state's ground water comes through
0:28:29 > 0:28:31springs like this.
0:28:36 > 0:28:39And what's so special about this place is that beneath me
0:28:39 > 0:28:42is a massive flooded cave complex,
0:28:42 > 0:28:45the so-called Floridan aquifer,
0:28:45 > 0:28:47through which all this water's flowed.
0:28:49 > 0:28:53And it's home to some of the most intriguing sinkholes in the state.
0:28:54 > 0:28:56- Hey!- Hi!- How's it going?
0:28:56 > 0:28:59You have picked the most beautiful spot.
0:28:59 > 0:29:02- Yeah, it's a remarkable area. - Fantastic.
0:29:02 > 0:29:06'Jarrod Jablonski and his team are among the most experienced
0:29:06 > 0:29:09'cave divers in the world, and they've been exploring
0:29:09 > 0:29:11'cave systems like these for decades.'
0:29:13 > 0:29:15Ah! That's better. Dry land.
0:29:15 > 0:29:18It's quite deep in there, though. I hadn't realised.
0:29:18 > 0:29:20Yeah, the water's actually a pretty good level.
0:29:20 > 0:29:23It varies a lot, depending on the drought conditions,
0:29:23 > 0:29:24how much rain we're getting.
0:29:25 > 0:29:28'What most people would find terrifying,
0:29:28 > 0:29:31'these divers find magical.'
0:29:31 > 0:29:34You've got this hidden world that no-one else
0:29:34 > 0:29:35but a few of you guys know about.
0:29:35 > 0:29:38Yeah, very much. We kind of enjoy that every now and then.
0:29:38 > 0:29:40You go down and you're looking around and it's just you
0:29:40 > 0:29:43and you start thinking about how few people in the world get that
0:29:43 > 0:29:45really special experience.
0:29:50 > 0:29:53'Even though Jarrod and his team are really experienced,
0:29:53 > 0:29:57'what they're about to do is still incredibly dangerous.'
0:29:59 > 0:30:02How many people have been killed, do you think, in these caves diving?
0:30:02 > 0:30:04Probably somewhere in the neighbourhood of 500 people
0:30:04 > 0:30:07have lost their lives exploring Florida caves.
0:30:07 > 0:30:10Most of those, certainly by a great majority, especially in the early
0:30:10 > 0:30:14years, were untrained open water divers, so really a very bad recipe.
0:30:14 > 0:30:16So they thought it was the same environment as the sea?
0:30:16 > 0:30:19Yeah, didn't know. Swim around in a beautiful place like this -
0:30:19 > 0:30:21as you said, it's beautiful and very benign looking.
0:30:21 > 0:30:24Then you go into the cave, which also looks at first benign,
0:30:24 > 0:30:26and then, if you don't know what you're doing, you can kick up the
0:30:26 > 0:30:30bottom conditions and you don't have a guideline, get lost pretty easily.
0:30:32 > 0:30:37'The entrance to this hidden, deadly Florida was just below us
0:30:37 > 0:30:40'and, whilst cave diving is a step too far for me,
0:30:40 > 0:30:44'I was keen just to look into its jaws.'
0:30:44 > 0:30:46- So where's the entrance then? - We're going to go down right here
0:30:46 > 0:30:50and we're going to go right in this way, which is going to descend down.
0:30:50 > 0:30:52We'll have about 6m deep and then we'll
0:30:52 > 0:30:55descend down to depth which will be about 20m deep.
0:30:55 > 0:30:57And you're going to head in that direction?
0:30:57 > 0:31:00In this direction towards a sinkhole called Pothole
0:31:00 > 0:31:02and then the next sinkhole, Olsen.
0:31:02 > 0:31:06There's a series of sinkholes that you can access through this conduit.
0:31:06 > 0:31:08All right, well, I'll see you off the premises, then.
0:31:08 > 0:31:10All right. Excuse me!
0:31:10 > 0:31:13I'll see how long I can hold my breath.
0:31:13 > 0:31:16- I like it.- Don't take me with you.
0:31:16 > 0:31:19All set? Good stuff!
0:31:46 > 0:31:49Well, they've gone.
0:31:49 > 0:31:53I can just see the last of them disappearing into the entrance.
0:31:53 > 0:31:55But, to be honest, it's not for me,
0:31:55 > 0:31:57so I'm going to follow them on dry land.
0:31:59 > 0:32:00As I headed ashore,
0:32:00 > 0:32:04Jarrod and team swam into the very throat of the underworld.
0:32:16 > 0:32:18For all their beauty,
0:32:18 > 0:32:20these labyrinths are lethal.
0:32:22 > 0:32:25All a diver has to do is kick up the sediment with a careless
0:32:25 > 0:32:29flick of a fin and the visibility will reduce to zero.
0:32:30 > 0:32:33That's how most people lose their lives down here.
0:32:39 > 0:32:43What I find amazing about these caves is how extensive they are.
0:32:46 > 0:32:49Divers have explored over 10km of them,
0:32:49 > 0:32:53but the caves' conduits and pore spaces of this aquifer
0:32:53 > 0:32:56stretch from one end of the state to the other.
0:33:04 > 0:33:08It's weird to think that they're right beneath my feet.
0:33:08 > 0:33:10Apparently, the first little bit's really tortuous,
0:33:10 > 0:33:13so if they manage to squeeze their way through that, then, according
0:33:13 > 0:33:17to this map, we should get the first indication of progress just up here.
0:33:21 > 0:33:24What I'm looking for is a small sinkhole -
0:33:24 > 0:33:27a place where the expanding cave has reached the point where
0:33:27 > 0:33:30its roof has failed, allowing the soils to fall down,
0:33:30 > 0:33:34creating a link between the worlds above and below.
0:33:40 > 0:33:43It's a sobering thought that those caves
0:33:43 > 0:33:46are expanding in all directions.
0:33:49 > 0:33:50And that's because all it takes
0:33:50 > 0:33:54to make the limestone dissolve is water,
0:33:54 > 0:33:56and there's plenty of that.
0:34:04 > 0:34:07So this is it.
0:34:14 > 0:34:17So this is Pothole sink, then.
0:34:17 > 0:34:20We've come from just about two minutes' walk away,
0:34:20 > 0:34:23from this Peacock Spring where the guys went in.
0:34:23 > 0:34:25And, at one point, the limestone would have been across, like this,
0:34:25 > 0:34:28but then what's happened is this bit's been dissolved down and then
0:34:28 > 0:34:32the wear of the soil at some point - maybe, I don't know, 10,000 years,
0:34:32 > 0:34:3520,000 years ago - the whole thing's just caved in on itself,
0:34:35 > 0:34:39collapsed, creating this hole, and you've got this.
0:34:39 > 0:34:41This is essentially a conduit.
0:34:41 > 0:34:44It's maybe 40, 50 foot down below.
0:34:44 > 0:34:48So, somewhere down there the guys are swimming past and supposedly
0:34:48 > 0:34:52what happens is you're going to see the bubbles as they go past.
0:34:52 > 0:34:53That's what we're waiting for.
0:35:05 > 0:35:08This is a nursery ground for sinkholes,
0:35:08 > 0:35:13the rock dissolving at a rate of around 4cm every thousand years.
0:35:17 > 0:35:18The void's getting bigger
0:35:18 > 0:35:22and ultimately the soil above falling into them,
0:35:22 > 0:35:25creating yet another sinkhole.
0:35:45 > 0:35:47There, there we go!
0:35:48 > 0:35:51So that means they're now right beneath us.
0:35:51 > 0:35:53The air's coming up, so that means they're safe so far.
0:35:53 > 0:35:56So, the thing is, they've got another hour to go before they can
0:35:56 > 0:36:00actually surface properly and have proper fresh air.
0:36:00 > 0:36:03At least they're safe. So far, so good.
0:36:03 > 0:36:05Now, onto the next bit.
0:36:10 > 0:36:13Dissolving limestone like this is known as karst,
0:36:13 > 0:36:16and it's not just confined to Florida.
0:36:19 > 0:36:22Limestone is common all over the world.
0:36:24 > 0:36:26There are pockets of it in the UK,
0:36:26 > 0:36:29but here sinkholes rarely make the news.
0:36:35 > 0:36:39What's special about Florida is the extent of the limestone
0:36:39 > 0:36:42and just how big the sinkholes are.
0:36:44 > 0:36:47You know, it's the kind of place that makes you contemplate
0:36:47 > 0:36:49Florida in a whole different way.
0:36:49 > 0:36:52If I hadn't have known there was a team of divers down there,
0:36:52 > 0:36:54this would just be another pond
0:36:54 > 0:36:57and another little patch of wood,
0:36:57 > 0:37:00but actually it's a gateway to the underworld,
0:37:00 > 0:37:04an underworld that stretches the length and breadth of Florida
0:37:04 > 0:37:06and an underworld that's killed
0:37:06 > 0:37:08and will kill again.
0:37:12 > 0:37:17So, at any point, I'm hoping to see this burst of bubbles
0:37:17 > 0:37:19and that'll be them, safe.
0:37:25 > 0:37:26Oh, yes!
0:37:26 > 0:37:27That must be them, look.
0:37:29 > 0:37:33Look at this. Yeah, here they come. Here they come.
0:37:33 > 0:37:35What a beautiful spot to come out, as well.
0:37:38 > 0:37:41There's something quite elegant about it.
0:37:46 > 0:37:50Really big sinkholes happen only rarely,
0:37:50 > 0:37:53but when they do, they make quite an impact.
0:38:00 > 0:38:05On the 8th of May 1981, the residents of Winter Park,
0:38:05 > 0:38:07close to Orlando, witnessed this.
0:38:12 > 0:38:15What started as a small hole soon developed into
0:38:15 > 0:38:17a 13m deep monster,
0:38:17 > 0:38:20some hundred metres wide.
0:38:20 > 0:38:23We watched a house slide in, we watched eight or nine cars
0:38:23 > 0:38:25slide in, we watched the swimming pool slide in.
0:38:25 > 0:38:28And you just sit there and watch it and you're powerless to help.
0:38:30 > 0:38:32Any thoughts about making it a lake?
0:38:32 > 0:38:34It will be a lake. We've already found that out yesterday.
0:38:34 > 0:38:37The engineers and that sort of people said it will be a lake and
0:38:37 > 0:38:39there's nothing we can do about that,
0:38:39 > 0:38:41so we just have a new lake in the city.
0:38:43 > 0:38:46When you're confronted with footage as dramatic as this,
0:38:46 > 0:38:48you have to ask, what triggered it?
0:38:50 > 0:38:52Why did it happen now?
0:38:53 > 0:38:57The ancient geology of this state, the limestone, isn't enough.
0:39:02 > 0:39:05First glance, it must seem as if nowhere's safe in Florida,
0:39:05 > 0:39:09but actually some places are more at risk than others.
0:39:09 > 0:39:13This map shows the locations of all the verified sinkholes
0:39:13 > 0:39:15and you can see how widespread they are across the state.
0:39:15 > 0:39:20But there's a real cluster of them here in West Central Florida -
0:39:20 > 0:39:23a sinkhole sweet spot, if you like.
0:39:23 > 0:39:26And Jeff Bush's place is just in there,
0:39:26 > 0:39:29right on the southern edge of that sweet spot.
0:39:35 > 0:39:36On the face of it,
0:39:36 > 0:39:39this clustering of sinkholes doesn't make much sense.
0:39:42 > 0:39:45Why are some parts of the state relatively safe
0:39:45 > 0:39:48and others much more prone to sinkholes...
0:39:49 > 0:39:54..even though they're all underlain by the same rock, limestone?
0:39:56 > 0:39:59So far, this search to understand what sinkholes
0:39:59 > 0:40:03are all about has focused on one material, rock.
0:40:03 > 0:40:08The thing is, voids in limestone open up ridiculously slowly,
0:40:08 > 0:40:10I mean, over thousands of years.
0:40:10 > 0:40:12So there's another material that we should consider,
0:40:12 > 0:40:15one that's far more mobile -
0:40:15 > 0:40:16soil.
0:40:26 > 0:40:30Geological mysteries seldom have just one culprit,
0:40:30 > 0:40:35and the material that covers this vast, unstable slab of limestone
0:40:35 > 0:40:39is rather more interesting than you might initially think.
0:40:39 > 0:40:42In fact, it determines how deadly sinkholes are.
0:40:44 > 0:40:47To understand why, you have to take a very close look
0:40:47 > 0:40:49at the soils around here.
0:40:52 > 0:40:54And the best way to do that is to take
0:40:54 > 0:40:56a trip down the Hillsborough River.
0:41:00 > 0:41:04The only problem with that is that the Hillsborough River
0:41:04 > 0:41:06is infested with alligators.
0:41:08 > 0:41:11I can't believe I'm risking my life for soil.
0:41:29 > 0:41:31There's one, there's one!
0:41:31 > 0:41:34There's one just there. Four, five-footer.
0:41:34 > 0:41:36Look at that.
0:41:36 > 0:41:39It's off, doesn't like me!
0:41:39 > 0:41:41I think it's more scared of me.
0:41:47 > 0:41:49The reason I'm out here on the Hillsborough River
0:41:49 > 0:41:52is that this area is almost sinkhole-free.
0:41:57 > 0:42:00But just downstream, it's a very different story.
0:42:03 > 0:42:06Look at this, a map of all the sinkholes in Florida.
0:42:06 > 0:42:10All these red squares here are crammed into West Central Florida.
0:42:10 > 0:42:15And then look, you get this kind of sinkhole-free area in here.
0:42:15 > 0:42:18To see that in more detail, we have to go to a different map.
0:42:18 > 0:42:19Look at this one here.
0:42:19 > 0:42:21This is the Hillsborough River,
0:42:21 > 0:42:23drifting in the Hillsborough River,
0:42:23 > 0:42:24just probably up here.
0:42:24 > 0:42:26We're just on that bend there.
0:42:26 > 0:42:30So, the thing is, in this area round here, there's just no sinkholes.
0:42:30 > 0:42:33But actually, to the west and to the east, it's sinkhole city.
0:42:33 > 0:42:36But to understand why there's no sinkholes here,
0:42:36 > 0:42:38we have to get to the shore.
0:42:50 > 0:42:54The limestone beneath me is the same as anywhere else in Florida
0:42:54 > 0:42:57and is riddled with caves and fissures.
0:42:57 > 0:43:00But it's the earth above that's different here.
0:43:01 > 0:43:04So what makes this place so safe?
0:43:06 > 0:43:09Time for a bit of old school geology.
0:43:13 > 0:43:17This is the instrument of a soil scientist, the auger.
0:43:17 > 0:43:21What you do is you clear a bit of ground
0:43:21 > 0:43:23and then you stick it in.
0:43:26 > 0:43:29Kind of just drill your way down through the vegetation.
0:43:33 > 0:43:36Can you hear the vegetation ripping?
0:43:36 > 0:43:40And then you get down into the soil, the topsoil,
0:43:40 > 0:43:43and as you get below that,
0:43:43 > 0:43:46it gets a little bit easier. In fact, it's easy now.
0:43:46 > 0:43:49So we can see what's underneath the topsoil.
0:43:49 > 0:43:51And what we see is,
0:43:51 > 0:43:54this is all soil but then this here,
0:43:54 > 0:43:57this is just sand. Look at that.
0:43:57 > 0:44:01Really coarse, loose sand.
0:44:01 > 0:44:05You can tell how coarse is it, cos when you chew it,
0:44:05 > 0:44:07it's quite gritty between your teeth.
0:44:07 > 0:44:10It's the sand that's key to there being
0:44:10 > 0:44:12so few sinkholes in this area.
0:44:12 > 0:44:16So the point is you've got this really loose, incohesive sand,
0:44:16 > 0:44:18and as the rainwater falls on it,
0:44:18 > 0:44:21to be honest, it just drains straight through.
0:44:21 > 0:44:23Straight through to the limestone below.
0:44:24 > 0:44:27This means that the soils never build up.
0:44:27 > 0:44:30The sands just fall into the voids as they form,
0:44:30 > 0:44:34so the closest thing you'll get to a big sinkhole round here
0:44:34 > 0:44:37is this rather innocuous puddle.
0:44:37 > 0:44:39The limestone underneath here would have started out
0:44:39 > 0:44:42something like that and then it's just been kind of dissolved down.
0:44:42 > 0:44:45And what you've had is you had the sand just kind of dripping, draining
0:44:45 > 0:44:49through it, and it just captures this, just a little patch of water.
0:44:49 > 0:44:52The limestone here is only a few feet beneath the
0:44:52 > 0:44:57thin sandy soil, which is constantly being washed down into it.
0:44:57 > 0:45:00Now, the thing is, these really aren't dangerous at all.
0:45:00 > 0:45:03There's hardly any chance of something like this collapsing.
0:45:06 > 0:45:10But a few miles down the river, you enter sinkhole alley.
0:45:11 > 0:45:13This is where the big sinkholes happen
0:45:13 > 0:45:17and the first thing you notice is the landscape's changed.
0:45:17 > 0:45:20We're out of the swamps and into an area rich in agriculture.
0:45:22 > 0:45:26During the winter, the fields around here are full of strawberries,
0:45:26 > 0:45:30making good use of the deep fertile soils and warm weather,
0:45:30 > 0:45:33so creating the fruit bowl of America.
0:45:37 > 0:45:40But I'm here in the off season and there's not a strawberry
0:45:40 > 0:45:45in sight, but that doesn't matter, as I'm only interested in the soil.
0:45:52 > 0:45:55You can see that's gone down much, much easier here.
0:45:55 > 0:45:58Now if I just pull that up now, let's see what we've got.
0:46:00 > 0:46:04OK, look, you can see it's much darker and it's still sandy,
0:46:04 > 0:46:08still pretty sandy but look, it's got some kind of strength to it,
0:46:08 > 0:46:11so that means there's some clay in there.
0:46:11 > 0:46:14Now if I went down another few metres or so,
0:46:14 > 0:46:16if I could be bothered going all the way down,
0:46:16 > 0:46:18I'd find something completely different.
0:46:18 > 0:46:20I'd find something like this.
0:46:20 > 0:46:22This is a clay and you can roll that.
0:46:22 > 0:46:25It's got some kind of strength. Look at that.
0:46:25 > 0:46:26Just not going to do anything.
0:46:26 > 0:46:29And it's that clay that turns out to be really important
0:46:29 > 0:46:31because that clay is really sticky.
0:46:31 > 0:46:35It's got its own strength and because of that,
0:46:35 > 0:46:37rather than as the water comes through,
0:46:37 > 0:46:40rather than it just washing the clay down into the sinkholes below,
0:46:40 > 0:46:45it stays there and it builds up, so you get a much thicker sequence
0:46:45 > 0:46:48of sediment and not just that, sediment with some strength to it.
0:46:50 > 0:46:53It's strong enough to bridge the holes in the limestone...
0:46:54 > 0:46:56..but not for ever.
0:46:58 > 0:47:01Sooner or later, the clay will give way,
0:47:01 > 0:47:04and that's what happened at Winter Park 30 years ago.
0:47:05 > 0:47:11Back then, no-one knew there was a void beneath this part of town...
0:47:11 > 0:47:16but the whole place stood on a clay trap door, just waiting to spring.
0:47:26 > 0:47:30It's a strange idea that a layer of clay could be
0:47:30 > 0:47:33the cause for a potentially deadly sinkhole.
0:47:33 > 0:47:36Let me show you how, in a rather homespun way.
0:47:38 > 0:47:39What happens is this...
0:47:39 > 0:47:42Above the voids in the limestone,
0:47:42 > 0:47:46a layer of sand can form with a muddy clay above it
0:47:46 > 0:47:48and more soil on top of that.
0:47:52 > 0:47:56So you get this layer cake of sand, mud and sand.
0:47:58 > 0:48:00And then this is the tricky part.
0:48:02 > 0:48:03OK.
0:48:04 > 0:48:07And it creates a void in the sub-surface. Look at that.
0:48:07 > 0:48:10And it's the clay layer that's really important.
0:48:10 > 0:48:14It's forming a strong bridge that's holding the rest of that sand
0:48:14 > 0:48:15up and not letting it collapse.
0:48:16 > 0:48:19But if the clay loses its strength,
0:48:19 > 0:48:23the bridge it created fails...suddenly.
0:48:25 > 0:48:27The whole thing just caves in.
0:48:27 > 0:48:29It's kind of like a trap door that's been pulled
0:48:29 > 0:48:32and that is called a collapse sinkhole,
0:48:32 > 0:48:34and it's absolutely deadly.
0:48:40 > 0:48:42But what triggers that failure?
0:48:51 > 0:48:54This might seem like an urban paradise,
0:48:54 > 0:48:57but appearances can be deceptive.
0:48:59 > 0:49:03Many people here wonder if they're sitting on top of a fragile
0:49:03 > 0:49:04clay bridge into the underworld.
0:49:06 > 0:49:11So, alongside the cement mixers trying to stabilise the foundations,
0:49:11 > 0:49:13this is a pretty typical scene.
0:49:20 > 0:49:22Geologists drilling.
0:49:29 > 0:49:32What they're looking for is clay beneath the sands,
0:49:32 > 0:49:36a potential bridge which could fail at any moment.
0:49:41 > 0:49:42And if that clay were to fail,
0:49:42 > 0:49:46the cause will be something we're very familiar with.
0:49:51 > 0:49:52Water.
0:49:53 > 0:49:57The state of Florida is greedy for water
0:49:57 > 0:50:01to keep the lawns looking green, to fill swimming pools...
0:50:03 > 0:50:08..and, most importantly of all, they need millions of gallons of water
0:50:08 > 0:50:11to grow the famous Florida strawberries and tomatoes.
0:50:13 > 0:50:16And the demand for all this water means the water table close
0:50:16 > 0:50:20to those fields can drop by over 50 feet in just a few days.
0:50:26 > 0:50:30And when the clay dries out, that lack of water can be the trigger.
0:50:35 > 0:50:38But that's just half the story.
0:50:39 > 0:50:43Water can also cause clay to fail because it becomes too wet.
0:50:45 > 0:50:48Florida famously is hurricane country.
0:50:52 > 0:50:57And when they hit, they can drop around a metre of water
0:50:57 > 0:50:59on the land in a matter of hours.
0:50:59 > 0:51:01Pretty bad out here now.
0:51:04 > 0:51:07The sheer wear of all that water on those fragile clay
0:51:07 > 0:51:10bridges can be enough to cause catastrophe...
0:51:12 > 0:51:15..so, ultimately, water,
0:51:15 > 0:51:19too much or too little, can be the trigger for a sinkhole collapse.
0:51:31 > 0:51:33But if water can be the trigger,
0:51:33 > 0:51:37then there's another reason why Florida's so prone to sinkholes.
0:51:40 > 0:51:46And when you've spent a bit of time here, it starts to become obvious.
0:51:46 > 0:51:47It's something to do with us.
0:51:48 > 0:51:51You know, the sinkholes, sinkholes have always been here.
0:51:51 > 0:51:55They're part of Florida's constantly evolving geology.
0:51:55 > 0:51:57But what's changed is us.
0:51:57 > 0:52:01I mean, our fondness for the sun, our expanding numbers,
0:52:01 > 0:52:03we're taking over more and more of the land.
0:52:11 > 0:52:16As the population of Florida grows, people need somewhere to live
0:52:16 > 0:52:19and that means we inevitably end up
0:52:19 > 0:52:22building on more sinkhole-prone land.
0:52:26 > 0:52:30It's easy to get the impression here that this is a state
0:52:30 > 0:52:31collapsing in on itself.
0:52:39 > 0:52:44There are almost 6,700 sinkhole-related claims each year,
0:52:44 > 0:52:46and the numbers are rising.
0:52:48 > 0:52:52But incredibly, they've only killed three people in the last
0:52:52 > 0:52:5540 years as few collapse without warning.
0:52:57 > 0:53:01What's far more typical is what happened at this restaurant.
0:53:01 > 0:53:04Just a few weeks back, the staff came in in the morning to find
0:53:04 > 0:53:08that cracks had appeared overnight, so they called in the engineers
0:53:08 > 0:53:12and basically they - look at it - condemned the whole building.
0:53:14 > 0:53:1730-odd people used to work here.
0:53:17 > 0:53:21But the thing is, because of those warning signs, there was no tragedy.
0:53:27 > 0:53:30Sinkholes may seem as old as the Earth itself,
0:53:30 > 0:53:34but for millions of years those deep voids have been forming
0:53:34 > 0:53:38incredibly slowly, collapsing down only rarely.
0:53:41 > 0:53:46In a sense, sinkhole-related damage comes from what we're doing...
0:53:46 > 0:53:51how we use our resources like water, and especially the fact that
0:53:51 > 0:53:55we're building on deep clay soils above voids like this.
0:54:06 > 0:54:11What happened to Jeff on the 28th of February, 2013,
0:54:11 > 0:54:13when a sinkhole opened up under his bedroom,
0:54:13 > 0:54:17sucking him deep into the earth, is mercifully rare.
0:54:17 > 0:54:21The bedroom floor just collapsed and my brother-in-law is in there.
0:54:21 > 0:54:23He's underneath the house.
0:54:23 > 0:54:25It was a freak combination of factors...
0:54:28 > 0:54:31..shaped by the geology of where he lived, certainly,
0:54:31 > 0:54:34but also due to tragic bad luck.
0:54:36 > 0:54:40What's clear is that Jeff's house lay in sinkhole alley.
0:54:42 > 0:54:45With its clay soil, it's prone to sudden sinkholes.
0:54:49 > 0:54:52A structural engineer who worked with the emergency workers
0:54:52 > 0:54:54that night was Bill Bracken.
0:54:56 > 0:54:59The sinkhole was centred exactly on the bedroom,
0:54:59 > 0:55:03and was contained completely within the footprint of the building,
0:55:03 > 0:55:06so it was not visible from the air, from outside.
0:55:06 > 0:55:08The only way to see that hole was to look in the window or
0:55:08 > 0:55:10be inside the structure.
0:55:16 > 0:55:18When they poked a remote camera through,
0:55:18 > 0:55:22they saw a neat hole in the concrete slab of Jeff's bedroom
0:55:22 > 0:55:25and a near vertical shaft over 20 feet deep.
0:55:30 > 0:55:34The sinkhole formed and the soil began to drop.
0:55:34 > 0:55:37Because you had the entire structure over the top of this,
0:55:37 > 0:55:40because it was completely contained inside the footprint
0:55:40 > 0:55:44of the building, when those soils began to drop,
0:55:44 > 0:55:49it was almost a flushing action that created a suction force,
0:55:49 > 0:55:54so it wasn't as though a hole opened up and things fell into it.
0:55:54 > 0:55:57It was almost as though, as the soil began to pull
0:55:57 > 0:55:59away from the underside and a void was being created,
0:55:59 > 0:56:01a suction force was being created as well.
0:56:01 > 0:56:04Just tugged at the bottom of the concrete base of the bedroom?
0:56:04 > 0:56:07So, it effectively pulled that slab down into the hole.
0:56:09 > 0:56:13When fire crews placed a listening device at the bottom,
0:56:13 > 0:56:15they were in for a shock -
0:56:15 > 0:56:17the sinkhole was still alive.
0:56:19 > 0:56:22By the time that they had deployed that, set that in there,
0:56:22 > 0:56:25placed that on, on the ground surface,
0:56:25 > 0:56:29walked back to the box, connected it and began receiving signals,
0:56:29 > 0:56:33they began to sense a tugging, if you will, on the cable.
0:56:33 > 0:56:36When they went to check,
0:56:36 > 0:56:39the listening device had already been pulled down into the ground,
0:56:39 > 0:56:42so over the next three days there was about 30 feet of cable that
0:56:42 > 0:56:46was draped out of the window and on the ground, and we watched that
0:56:46 > 0:56:5030 feet of cable reduce down to about six-to-eight feet of cable.
0:56:50 > 0:56:53So we knew that that sand was still in an excited state.
0:56:53 > 0:56:55It was still pulling down.
0:56:55 > 0:56:58Everything that landed on it was immediately pulled down in,
0:56:58 > 0:57:03so we knew that whatever had gone into that was down at least
0:57:03 > 0:57:0530, 35 feet.
0:57:05 > 0:57:08SIREN BLARES
0:57:12 > 0:57:15The one certainty is that when the emergency services
0:57:15 > 0:57:19sent down a probe to look for signs of life, they found none.
0:57:26 > 0:57:29All that's left now is a family trying to come to terms with
0:57:29 > 0:57:32the freak tragic events of that night.
0:57:35 > 0:57:40They told us that they were not for sure how far down he was
0:57:40 > 0:57:45and that the ground around everything was just too...
0:57:46 > 0:57:51..too unsafe, too unstable for them to do anything.
0:57:51 > 0:57:53They could not risk anybody else's life.
0:58:16 > 0:58:19It makes me sick, just the way it looks.
0:58:19 > 0:58:24Overgrown with grass, nobody taking care of anything. And there's a...
0:58:25 > 0:58:27My brother's down there still.