Swallowed by a Sinkhole Horizon

Seffner, Florida.

A place where the earth opened up...

..and killed a man.

Lazy Lanes, this place is called.

It just seems so ordinary, so normal.

DOG BARKS

I mean, look at this, "Beware of the dog."

And yet this whole estate sits above a trap door into the hidden Florida.

'The bedroom floor just collapsed

'and my brother-in-law is underneath the house.'

The trap door into that hidden Florida opened here,

in February 2013...

..creating a sinkhole.

Without warning, it swallowed everything in this bedroom,

and for those who saw it, it was something they'll never forget.

It's like this thing was alive.

You know, it was turning, moving around,

making noises, you know, almost like a growl.

Sinkholes don't just happen in Florida.

They're occurring all over the world.

I want to find out why sinkholes form...

..what this underworld is really like.

You've not been down there?

Never been in there, didn't even know it existed.

And why some are deadly.

Until last spring, a house stood here.

A home lived in by two brothers,

Jeff and Jeremy Bush, along with their families.

My bedroom was right here.

So, your bedroom was here?

Yeah, my bedroom was right here in the front.

And you walked in through the front door, and there was a living room.

Yeah, so, you went in and then the living room was on the right?

Yeah, living room was on the right, kitchen behind my bedroom,

and then the dining room and then my brother's room.

That was... So it was on the far right-hand side.

-And then there was another bedroom, it was Janell's...

-Oh, right.

It was Janell's bedroom.

It was a normal night, I guess?

It was, it was normal.

I had just got home from work,

come in about ten, 10.30,

and told everybody good night and went to bed.

And then that's when it happened.

My wife turned the light on, and...

..as I was getting ready to walk in the door,

she turned the light on and all you could see was this big hole.

It sounded like a car hit the house,

but the house didn't move, um, nothing moved.

The walls didn't move, nothing.

Pictures were still hanging on the wall, everything.

It just was a loud crash.

That's a scream I'll never forget.

Um...

He just kept saying, "Help me, somebody help me."

SIREN WAILS

That scream was the last anyone heard of Jeff Bush.

No-one had any idea what was happening,

just that he was in trouble.

It was a 911 call

and it said that a family member had fallen underneath the house.

The first one there was Deputy Duvall.

I guess my first idea

was it may have been, like, an accident-type,

like, somebody was trying to renovate their house

and something happened with the floor,

it was rotten, you know, something of that sort.

He rushed straight in.

He went straight in the house,

and he pulled a couple of people out of the house,

made them get out of the house.

Everybody was screaming and kind of running around.

As soon as I saw them, I knew that it wasn't just somebody

that had fallen into the floor by an accident, you know?

I knew just from their reactions

that it was something a lot more significant.

I looked inside the room. There was nothing.

All you could do was smell fresh dirt.

It took his whole bedroom.

The only piece of concrete that was left was by the door.

It took his bed, his dresser, his TV and everything down in the hole.

The door was open and when I went through the house,

everything looked like it was normal.

You know, the floor was intact and the lights were on,

the power was working.

Nothing out of the ordinary - but I went to the bedroom,

and the door was open, and as soon as I turned and looked in,

there was nothing in the bedroom. It was just a giant hole.

Jeff had been pulled down into the underworld by a sinkhole.

All I could see was the tip of his box spring,

the tip of his bed frame and his mattress, and that was it.

You didn't see Jeff at that point?

Couldn't see Jeff, I thought I... I thought I could hear him

-yelling for me to help him.

-Yeah, and so you just...

I just jumped in there and started digging.

You want to take a minute?

Jeremy was... If you're walking into the actual bedroom,

he's pretty much right there between the door and the centre of the hole.

I started digging by my hands,

and I was yelling and screaming for him,

and yelling for my father-in-law to get a shovel

and a flashlight, so I could see.

So I grabbed hold of his bed and tried moving his bed,

and it wouldn't move

and I broke the bedframe in half trying to get it out.

Then I started... He got me the shovel and I started digging.

And they were real close. You've seen Jeremy, you've seen Jeff.

And they worked together, they did everything together.

They played video games together, they...

They were... They were brothers, they were tight brothers.

Jeff Bush had been swallowed alive,

and the sinkhole came close to swallowing his brother, Jeremy, too.

The ground was still falling as I was in the hole

and the concrete was moving and breaking, still.

-So, you were on your way down?

-I was on my way down.

-And you didn't even notice it?

-I wasn't paying attention to it at all.

I was just trying to get my brother out.

If you can imagine an hourglass, the funnel inside of an hourglass,

you've got the deeper portion,

then you've got the out... the outermost wall.

Jeremy was in the middle and while it was sinking,

it was also expanding out

because everything on the outside was filling in the void.

I know I didn't want to come out the hole,

I wanted to pull the deputy in there with me to help me dig him out,

-because nobody was helping.

-Of course.

Those who saw it forming will never forget it.

When everybody asks me about it,

I tell them it's like this thing was alive,

and when I say that it was eating, it literally...

Things, the furniture, it was still sinking,

it was still going into the ground.

And, you know, it was kind of, kind of turning, moving around,

making noises, you know,

almost like a growl, and it just... Like something was alive.

Jeremy was seconds from being sucked down

to the same terrible fate as his brother

when Deputy Duvall saved him.

He grabbed me by my arm and snatched me out of the hole,

and that was the last time that anybody went back in the house.

When the sun came up, everything seemed normal from the outside.

All anyone knew was that Jeff Bush was gone,

his body never to be discovered.

I found meeting Jeremy and hearing what happened to his brother

deeply disturbing and unsettling.

Such a horrible thought, isn't it?

That idea of the ground opening up and literally swallowing you alive,

it's just... It's the stuff of nightmares.

My science, geology, tries to give answers to why things happen

and hopefully save lives.

And one of the reasons I've become so interested in sinkholes right now

is that I've noticed more of them in the news.

In the last few years,

they have been captured as far afield as China...

..and Guatemala.

They create the fear that the ground beneath our feet

could open up into an unseen world at any time.

Incredibly, this young girl survived.

But it's in Florida

where the fear of dropping into the underworld is greatest.

A few months after Jeff Bush died,

over 100 people were saved from this resort complex

near Disney World just before it collapsed.

The dubious honour of being called the sinkhole capital of the world

costs the state of Florida hundreds of millions of dollars a year.

So at least lawyers are getting something out of it.

Look at that.

As soon as homeowners see some kind of crack in their house,

they're encouraged to phone these sinkhole attorneys.

And they ply their trade

up and down the highway here with all their billboards.

I've never seen a sinkhole before, not up close,

but to really understand them you can't just look at the surface.

You have to see what's happening underneath.

If you really want to know what the Florida underworld is like, you've got to descend into it.

On the rope!

There's one thing I could tell straightaway -

the rock I'm descending through is very familiar.

It's one of the most common rock types in the world

and obviously one of the most useful - it's a raw ingredient for cement.

But it's also the maker of these fantastic subterranean worlds.

If you were walking above, you'd never know this was here.

A great void, taking shape beneath the surface of the earth.

Professor Jason Polk has been trying to understand what it is that makes them collapse.

He estimates that the caves started forming millions of years ago.

That's the last kind of part of it, breaching to the surface?

Eventually the rock becomes so thin

that a large collapse can occur instantaneously.

And a lot of the sinkholes you see in Florida where

you have those instant collapses are where it's thin rock and thick soil.

So what's your best bet about how old that collapse was, when did daylight come in here?

Best guess is from work we've done with the sediments where

we've done radiocarbon dating to actually see how old these sediments are,

and we know that they are at least 10,000 years old,

-so that collapse is probably 10,000 years old at a minimum.

-Yeah.

It may look like the sinkhole's dead, but it's anything but.

Ever since the roof fell in, the cave has been filling with sediment.

But then three years ago, that sediment disappeared.

So 2010 then, it was up to there, is that right?

You would have been under sediment.

And just within minutes, hours - straight down?

-Just almost immediately.

-And where, down...down here?

Straight down here in this hole, which is the pathway to some

unknown void below where all of this sediment is washed down and continues to wash down.

-You've not been down there?

-Never been in there, didn't even know it existed.

-God!

So do you reckon it's...? I mean, are you slightly freaked out

that underneath we know there's a big hole?

It feels solid but we know that if we'd have been here a few years ago when this happened, we'd be...

-We'd be down there.

-We'd be down below in the unknown.

-Wow, that's a bit scary.

-We should go.

I don't like that, actually.

That gaping hole showed how alive this sinkhole still is.

I was very glad to leave.

I've been told that if I want to get a sense of just what

a problem sinkholes really are here, there's somewhere I should go.

This estate is just north of Tampa.

In many ways, it's a pretty unremarkable place.

Full of retirees chasing the dream of all year round sun.

It all looks so perfect, doesn't it? I mean, look at it.

But behind the facade, virtually all of these houses have got

structural problems and cracks and got people going to

bed at night really not sure if those sounds that they're

hearing is going to be another hole appearing underneath them.

So instead of spending their time making extensions and patios...

..the builders round here are pouring hundreds of tonnes of cement beneath

the foundations of these houses in an attempt to stabilise them.

'I went to visit one of the long-standing residents, Darlene Denaro.'

So how many of these houses would you say have had problems

or are getting problems now?

Erm...well, she has one, and in one down there on that side...

-By the white car?

-By the white car,

-and then across the street there's one that was fixed.

-Right.

Yeah, then this one and this one,

and then she suspects she has a sinkhole.

Which one, which one, the one with the green car?

-No, on the other side.

-Other side of that?

Yeah, and then around the corner, where Louis lives,

he lives on this side right here and he had, they had...

It's one, two, three and then one down here, he had a humungous one.

I think he had pretty close to 70 truck loads.

So you're kind of in the centre, really, of this... Sorry, well,

yeah, I was thinking, I mean, that...

I feel like I'm in... Yeah, I'm in the centre.

For decades, new housing has been springing up all across Florida.

Built on what was once rough farmland,

they seemed to offer a golden future for their new inhabitants.

But now I get the sense that many of them

are gripped by a collective fear of what might lie beneath.

-When you moved here, was anything mentioned about sinkholes here?

-Nothing.

-What was it before?

-Farmland, wet farmland, swampland.

Everybody who's... The old Florida people that had been

here for years, it was a very wet, soggy, swamp piece of property.

We would have never moved here, never.

-If you'd known.

-No.

What's happening on this estate is not that unusual -

sinkholes ruining the American Dream.

Look at this. Here we've got one.

For a lot of people, this is reality now,

trying to fill in this huge void underneath their houses.

I detected a real fear on this estate.

After all, Jeff Bush lived just an hour down the road.

But how justified are those fears?

It's very hard to get a sense of how many sinkholes there are in this state.

And just what a threat they are to the people living here.

So I'm going to try and get a rather higher perspective on the problem.

RADIO CHATTER

It's going to be so good to get up top because, I mean, to be

honest, on the ground, it's quite a tedious landscape.

It's very flat, there's lots of trees and tarmac that obscure

the views, but I think that from here, everything will become clear.

So we're going to go up about 500 feet, is that right?

-Above sea level.

-Right.

What I'm looking for is evidence of ancient sinkholes,

depressions in the land that have formed into lakes.

You can start to see some now. You see that over there?

Just a pockmark, a series of little lakes.

Like the whole place now is just lake land, everywhere you can see.

It's just little pockets, like just here.

Looks like the whole place is a giant golf course with kind of water hazards.

This is pretty much Florida, I guess.

Each one of those lakes beneath us

started as a hole into a limestone cavern beneath.

It's quite interesting, there's this intricate anatomy to them.

Some of them are just, just perfect circles,

and that's just one sinkhole, but others you can see,

it's like four or five have all joined together.

So the whole thing is just pockmarked as far as you can see,

filled with water, so you get these ornamental water features

that people build their houses and jetties around.

They're really sought-after.

But the thing is, potentially, they're lethal. I mean, these things can open up

and you're left with a hole in the ground.

It's reckoned there are several thousand of these sinkhole

lakes across the state of Florida, several thousand of them!

And that's just a part of it.

I tried to get a figure on just how many sinkholes there are in the state.

One county claims to have over 6,000 of them,

but the truth is, no-one really knows.

They've literally stopped counting.

To understand why,

you have to travel back to the very origins of the state itself.

Head a couple of miles out to sea and you can observe the whole process beginning.

Tens of millions of years ago,

modern Florida emerged as shallow seas just like this receded.

Warm seas that once teemed with marine life.

Quite nice, nice temperature.

No need for a snorkel and flippers.

It's amazingly shallow, even two miles off the shore.

For a wee walk in the Florida Keys.

What I'm standing on is the remains of the marine life that

inhabited this warm shallow coastline.

And the remains of those creatures

form a mud made from carbonates.

When those creatures die and decompose,

they turn into this, carbonate mud.

It's amazing to think this stuff is just the smashed-up

hard parts of millions of sea creatures.

And yet this is Florida in the making.

Over hundreds of thousands of years it gets compressed

into limestone, the rock that virtually all of Florida is made of.

An entire peninsula, around 500 miles north to south

and over 160 east to west.

And almost every inch of that is made of marine organisms.

And that's the key to sinkholes, really,

because what can be created can also be destroyed.

This is the bedrock of Florida stripped bare -

an old quarry at Windley Key.

If this looks solid and unchanging, it isn't.

CREAKING

Something has been eating it away and still is.

The thing is, everywhere you look, this rock is being destroyed.

I mean, look at this bit here.

It's been eroded away.

These are kind of miniature sinkholes, really.

And what's created them is just rain.

The rain is falling down and dissolving them away.

But to eat away the limestone, that rain has to change.

Pure water has a pH of around about seven,

but as it falls through the atmosphere it picks up

carbon dioxide molecules that turns it into a very weak acid,

carbonic acid.

And the other thing is that if it hits, here,

rotting vegetation and soil,

then that pH drops even more.

So, look at this. This is a pH meter and it's reading seven.

Now, if I stick it into this soil here,

it's gone down - 6.6, 5.3.

So that is really quite dramatically more acidic.

It's that acidity over thousands, tens of thousands of years,

that basically eats out those huge caves.

And if you don't believe me that acid can dissolve away rock,

I've got a little bit of acid.

It's hydrochloric acid, but it's quite dilute.

Look, if I pour it on my skin, it does nothing.

But if I pour it on this fossil coral...

look at that.

It's just going crazy.

One moment you've got calcium carbonate,

next moment it's all fizzed back to carbon dioxide.

And it's this process that's very slowly dissolving the whole state.

One of the things that drew me

to geology is how it makes you see the certainties

of the world we've created, the human planet, rather differently.

There are almost 20 million people living in Florida

and the population's growing rapidly.

Driving along the streets of somewhere like Miami, you feel

as if you're in one of the safest, most modern places in the world.

It's all built on rocks that are being dissolved by water.

Every drop that falls from the sky,

every drop that sinks through the ground, is turning to acid,

and that acid is very slowly dissolving the whole state.

To witness it in action, you have to leave the urban sprawl

and head out into the old Florida,

the Florida that existed before people ever set foot here.

Step just a few feet from the freeway

and you're into this primeval land of swamp and alligator.

It's a world I'm entering in search of something rather special.

I'm off to meet up with a remarkable team of explorers

who will really be able to show me

why there are so many sinkholes in Florida.

This is Peacock Springs.

It's an alien place.

It's a part of Florida that most tourists,

even most residents, never see.

It's like a lost world.

The water is just so clear.

And so it should be, really,

because this is Florida's lifeblood.

Something like 95% of the state's ground water comes through

springs like this.

And what's so special about this place is that beneath me

is a massive flooded cave complex,

the so-called Floridan aquifer,

through which all this water's flowed.

And it's home to some of the most intriguing sinkholes in the state.

-Hey!

-Hi!

-How's it going?

You have picked the most beautiful spot.

-Yeah, it's a remarkable area.

-Fantastic.

'Jarrod Jablonski and his team are among the most experienced

'cave divers in the world, and they've been exploring

'cave systems like these for decades.'

Ah! That's better. Dry land.

It's quite deep in there, though. I hadn't realised.

Yeah, the water's actually a pretty good level.

It varies a lot, depending on the drought conditions,

how much rain we're getting.

'What most people would find terrifying,

'these divers find magical.'

You've got this hidden world that no-one else

but a few of you guys know about.

Yeah, very much. We kind of enjoy that every now and then.

You go down and you're looking around and it's just you

and you start thinking about how few people in the world get that

really special experience.

'Even though Jarrod and his team are really experienced,

'what they're about to do is still incredibly dangerous.'

How many people have been killed, do you think, in these caves diving?

Probably somewhere in the neighbourhood of 500 people

have lost their lives exploring Florida caves.

Most of those, certainly by a great majority, especially in the early

years, were untrained open water divers, so really a very bad recipe.

So they thought it was the same environment as the sea?

Yeah, didn't know. Swim around in a beautiful place like this -

as you said, it's beautiful and very benign looking.

Then you go into the cave, which also looks at first benign,

and then, if you don't know what you're doing, you can kick up the

bottom conditions and you don't have a guideline, get lost pretty easily.

'The entrance to this hidden, deadly Florida was just below us

'and, whilst cave diving is a step too far for me,

'I was keen just to look into its jaws.'

-So where's the entrance then?

-We're going to go down right here

and we're going to go right in this way, which is going to descend down.

We'll have about 6m deep and then we'll

descend down to depth which will be about 20m deep.

And you're going to head in that direction?

In this direction towards a sinkhole called Pothole

and then the next sinkhole, Olsen.

There's a series of sinkholes that you can access through this conduit.

All right, well, I'll see you off the premises, then.

All right. Excuse me!

I'll see how long I can hold my breath.

-I like it.

-Don't take me with you.

All set? Good stuff!

Well, they've gone.

I can just see the last of them disappearing into the entrance.

But, to be honest, it's not for me,

so I'm going to follow them on dry land.

As I headed ashore,

Jarrod and team swam into the very throat of the underworld.

For all their beauty,

these labyrinths are lethal.

All a diver has to do is kick up the sediment with a careless

flick of a fin and the visibility will reduce to zero.

That's how most people lose their lives down here.

What I find amazing about these caves is how extensive they are.

Divers have explored over 10km of them,

but the caves' conduits and pore spaces of this aquifer

stretch from one end of the state to the other.

It's weird to think that they're right beneath my feet.

Apparently, the first little bit's really tortuous,

so if they manage to squeeze their way through that, then, according

to this map, we should get the first indication of progress just up here.

What I'm looking for is a small sinkhole -

a place where the expanding cave has reached the point where

its roof has failed, allowing the soils to fall down,

creating a link between the worlds above and below.

It's a sobering thought that those caves

are expanding in all directions.

And that's because all it takes

to make the limestone dissolve is water,

and there's plenty of that.

So this is it.

So this is Pothole sink, then.

We've come from just about two minutes' walk away,

from this Peacock Spring where the guys went in.

And, at one point, the limestone would have been across, like this,

but then what's happened is this bit's been dissolved down and then

the wear of the soil at some point - maybe, I don't know, 10,000 years,

20,000 years ago - the whole thing's just caved in on itself,

collapsed, creating this hole, and you've got this.

This is essentially a conduit.

It's maybe 40, 50 foot down below.

So, somewhere down there the guys are swimming past and supposedly

what happens is you're going to see the bubbles as they go past.

That's what we're waiting for.

This is a nursery ground for sinkholes,

the rock dissolving at a rate of around 4cm every thousand years.

The void's getting bigger

and ultimately the soil above falling into them,

creating yet another sinkhole.

There, there we go!

So that means they're now right beneath us.

The air's coming up, so that means they're safe so far.

So, the thing is, they've got another hour to go before they can

actually surface properly and have proper fresh air.

At least they're safe. So far, so good.

Now, onto the next bit.

Dissolving limestone like this is known as karst,

and it's not just confined to Florida.

Limestone is common all over the world.

There are pockets of it in the UK,

but here sinkholes rarely make the news.

What's special about Florida is the extent of the limestone

and just how big the sinkholes are.

You know, it's the kind of place that makes you contemplate

Florida in a whole different way.

If I hadn't have known there was a team of divers down there,

this would just be another pond

and another little patch of wood,

but actually it's a gateway to the underworld,

an underworld that stretches the length and breadth of Florida

and an underworld that's killed

and will kill again.

So, at any point, I'm hoping to see this burst of bubbles

and that'll be them, safe.

Oh, yes!

That must be them, look.

Look at this. Yeah, here they come. Here they come.

What a beautiful spot to come out, as well.

There's something quite elegant about it.

Really big sinkholes happen only rarely,

but when they do, they make quite an impact.

On the 8th of May 1981, the residents of Winter Park,

close to Orlando, witnessed this.

What started as a small hole soon developed into

a 13m deep monster,

some hundred metres wide.

We watched a house slide in, we watched eight or nine cars

slide in, we watched the swimming pool slide in.

And you just sit there and watch it and you're powerless to help.

Any thoughts about making it a lake?

It will be a lake. We've already found that out yesterday.

The engineers and that sort of people said it will be a lake and

there's nothing we can do about that,

so we just have a new lake in the city.

When you're confronted with footage as dramatic as this,

you have to ask, what triggered it?

Why did it happen now?

The ancient geology of this state, the limestone, isn't enough.

First glance, it must seem as if nowhere's safe in Florida,

but actually some places are more at risk than others.

This map shows the locations of all the verified sinkholes

and you can see how widespread they are across the state.

But there's a real cluster of them here in West Central Florida -

a sinkhole sweet spot, if you like.

And Jeff Bush's place is just in there,

right on the southern edge of that sweet spot.

On the face of it,

this clustering of sinkholes doesn't make much sense.

Why are some parts of the state relatively safe

and others much more prone to sinkholes...

..even though they're all underlain by the same rock, limestone?

So far, this search to understand what sinkholes

are all about has focused on one material, rock.

The thing is, voids in limestone open up ridiculously slowly,

I mean, over thousands of years.

So there's another material that we should consider,

one that's far more mobile -

soil.

Geological mysteries seldom have just one culprit,

and the material that covers this vast, unstable slab of limestone

is rather more interesting than you might initially think.

In fact, it determines how deadly sinkholes are.

To understand why, you have to take a very close look

at the soils around here.

And the best way to do that is to take

a trip down the Hillsborough River.

The only problem with that is that the Hillsborough River

is infested with alligators.

I can't believe I'm risking my life for soil.

There's one, there's one!

There's one just there. Four, five-footer.

Look at that.

It's off, doesn't like me!

I think it's more scared of me.

The reason I'm out here on the Hillsborough River

is that this area is almost sinkhole-free.

But just downstream, it's a very different story.

Look at this, a map of all the sinkholes in Florida.

All these red squares here are crammed into West Central Florida.

And then look, you get this kind of sinkhole-free area in here.

To see that in more detail, we have to go to a different map.

Look at this one here.

This is the Hillsborough River,

drifting in the Hillsborough River,

just probably up here.

We're just on that bend there.

So, the thing is, in this area round here, there's just no sinkholes.

But actually, to the west and to the east, it's sinkhole city.

But to understand why there's no sinkholes here,

we have to get to the shore.

The limestone beneath me is the same as anywhere else in Florida

and is riddled with caves and fissures.

But it's the earth above that's different here.

So what makes this place so safe?

Time for a bit of old school geology.

This is the instrument of a soil scientist, the auger.

What you do is you clear a bit of ground

and then you stick it in.

Kind of just drill your way down through the vegetation.

Can you hear the vegetation ripping?

And then you get down into the soil, the topsoil,

and as you get below that,

it gets a little bit easier. In fact, it's easy now.

So we can see what's underneath the topsoil.

And what we see is,

this is all soil but then this here,

this is just sand. Look at that.

Really coarse, loose sand.

You can tell how coarse is it, cos when you chew it,

it's quite gritty between your teeth.

It's the sand that's key to there being

so few sinkholes in this area.

So the point is you've got this really loose, incohesive sand,

and as the rainwater falls on it,

to be honest, it just drains straight through.

Straight through to the limestone below.

This means that the soils never build up.

The sands just fall into the voids as they form,

so the closest thing you'll get to a big sinkhole round here

is this rather innocuous puddle.

The limestone underneath here would have started out

something like that and then it's just been kind of dissolved down.

And what you've had is you had the sand just kind of dripping, draining

through it, and it just captures this, just a little patch of water.

The limestone here is only a few feet beneath the

thin sandy soil, which is constantly being washed down into it.

Now, the thing is, these really aren't dangerous at all.

There's hardly any chance of something like this collapsing.

But a few miles down the river, you enter sinkhole alley.

This is where the big sinkholes happen

and the first thing you notice is the landscape's changed.

We're out of the swamps and into an area rich in agriculture.

During the winter, the fields around here are full of strawberries,

making good use of the deep fertile soils and warm weather,

so creating the fruit bowl of America.

But I'm here in the off season and there's not a strawberry

in sight, but that doesn't matter, as I'm only interested in the soil.

You can see that's gone down much, much easier here.

Now if I just pull that up now, let's see what we've got.

OK, look, you can see it's much darker and it's still sandy,

still pretty sandy but look, it's got some kind of strength to it,

so that means there's some clay in there.

Now if I went down another few metres or so,

if I could be bothered going all the way down,

I'd find something completely different.

I'd find something like this.

This is a clay and you can roll that.

It's got some kind of strength. Look at that.

Just not going to do anything.

And it's that clay that turns out to be really important

because that clay is really sticky.

It's got its own strength and because of that,

rather than as the water comes through,

rather than it just washing the clay down into the sinkholes below,

it stays there and it builds up, so you get a much thicker sequence

of sediment and not just that, sediment with some strength to it.

It's strong enough to bridge the holes in the limestone...

..but not for ever.

Sooner or later, the clay will give way,

and that's what happened at Winter Park 30 years ago.

Back then, no-one knew there was a void beneath this part of town...

but the whole place stood on a clay trap door, just waiting to spring.

It's a strange idea that a layer of clay could be

the cause for a potentially deadly sinkhole.

Let me show you how, in a rather homespun way.

What happens is this...

Above the voids in the limestone,

a layer of sand can form with a muddy clay above it

and more soil on top of that.

So you get this layer cake of sand, mud and sand.

And then this is the tricky part.

OK.

And it creates a void in the sub-surface. Look at that.

And it's the clay layer that's really important.

It's forming a strong bridge that's holding the rest of that sand

up and not letting it collapse.

But if the clay loses its strength,

the bridge it created fails...suddenly.

The whole thing just caves in.

It's kind of like a trap door that's been pulled

and that is called a collapse sinkhole,

and it's absolutely deadly.

But what triggers that failure?

This might seem like an urban paradise,

but appearances can be deceptive.

Many people here wonder if they're sitting on top of a fragile

clay bridge into the underworld.

So, alongside the cement mixers trying to stabilise the foundations,

this is a pretty typical scene.

Geologists drilling.

What they're looking for is clay beneath the sands,

a potential bridge which could fail at any moment.

And if that clay were to fail,

the cause will be something we're very familiar with.

Water.

The state of Florida is greedy for water

to keep the lawns looking green, to fill swimming pools...

..and, most importantly of all, they need millions of gallons of water

to grow the famous Florida strawberries and tomatoes.

And the demand for all this water means the water table close

to those fields can drop by over 50 feet in just a few days.

And when the clay dries out, that lack of water can be the trigger.

But that's just half the story.

Water can also cause clay to fail because it becomes too wet.

Florida famously is hurricane country.

And when they hit, they can drop around a metre of water

on the land in a matter of hours.

Pretty bad out here now.

The sheer wear of all that water on those fragile clay

bridges can be enough to cause catastrophe...

..so, ultimately, water,

too much or too little, can be the trigger for a sinkhole collapse.

But if water can be the trigger,

then there's another reason why Florida's so prone to sinkholes.

And when you've spent a bit of time here, it starts to become obvious.

It's something to do with us.

You know, the sinkholes, sinkholes have always been here.

They're part of Florida's constantly evolving geology.

But what's changed is us.

I mean, our fondness for the sun, our expanding numbers,

we're taking over more and more of the land.

As the population of Florida grows, people need somewhere to live

and that means we inevitably end up

building on more sinkhole-prone land.

It's easy to get the impression here that this is a state

collapsing in on itself.

There are almost 6,700 sinkhole-related claims each year,

and the numbers are rising.

But incredibly, they've only killed three people in the last

40 years as few collapse without warning.

What's far more typical is what happened at this restaurant.

Just a few weeks back, the staff came in in the morning to find

that cracks had appeared overnight, so they called in the engineers

and basically they - look at it - condemned the whole building.

30-odd people used to work here.

But the thing is, because of those warning signs, there was no tragedy.

Sinkholes may seem as old as the Earth itself,

but for millions of years those deep voids have been forming

incredibly slowly, collapsing down only rarely.

In a sense, sinkhole-related damage comes from what we're doing...

how we use our resources like water, and especially the fact that

we're building on deep clay soils above voids like this.

What happened to Jeff on the 28th of February, 2013,

when a sinkhole opened up under his bedroom,

sucking him deep into the earth, is mercifully rare.

The bedroom floor just collapsed and my brother-in-law is in there.

He's underneath the house.

It was a freak combination of factors...

..shaped by the geology of where he lived, certainly,

but also due to tragic bad luck.

What's clear is that Jeff's house lay in sinkhole alley.

With its clay soil, it's prone to sudden sinkholes.

A structural engineer who worked with the emergency workers

that night was Bill Bracken.

The sinkhole was centred exactly on the bedroom,

and was contained completely within the footprint of the building,

so it was not visible from the air, from outside.

The only way to see that hole was to look in the window or

be inside the structure.

When they poked a remote camera through,

they saw a neat hole in the concrete slab of Jeff's bedroom

and a near vertical shaft over 20 feet deep.

The sinkhole formed and the soil began to drop.

Because you had the entire structure over the top of this,

because it was completely contained inside the footprint

of the building, when those soils began to drop,

it was almost a flushing action that created a suction force,

so it wasn't as though a hole opened up and things fell into it.

It was almost as though, as the soil began to pull

away from the underside and a void was being created,

a suction force was being created as well.

Just tugged at the bottom of the concrete base of the bedroom?

So, it effectively pulled that slab down into the hole.

When fire crews placed a listening device at the bottom,

they were in for a shock -

the sinkhole was still alive.

By the time that they had deployed that, set that in there,

placed that on, on the ground surface,

walked back to the box, connected it and began receiving signals,

they began to sense a tugging, if you will, on the cable.

When they went to check,

the listening device had already been pulled down into the ground,

so over the next three days there was about 30 feet of cable that

was draped out of the window and on the ground, and we watched that

30 feet of cable reduce down to about six-to-eight feet of cable.

So we knew that that sand was still in an excited state.

It was still pulling down.

Everything that landed on it was immediately pulled down in,

so we knew that whatever had gone into that was down at least

30, 35 feet.

SIREN BLARES

The one certainty is that when the emergency services

sent down a probe to look for signs of life, they found none.

All that's left now is a family trying to come to terms with

the freak tragic events of that night.

They told us that they were not for sure how far down he was

and that the ground around everything was just too...

..too unsafe, too unstable for them to do anything.

They could not risk anybody else's life.

It makes me sick, just the way it looks.

Overgrown with grass, nobody taking care of anything. And there's a...

My brother's down there still.