Original subtitles

NARRATOR: They can occur suddenly

and catastrophically.

WOMAN: Oh, my God, do you see it?

NARRATOR: They have swallowed cars,

animals

and people.

They have destroyed homes.

MAN: Our house and also our neighbor's house

is completely gone.

NARRATOR: And for an unlucky few have become graves.

These are sinkholes.

All across the globe,

enormous sinkholes have cracked the earth's surface

like an eggshell, devouring every type of terrain

with a destructive force that defies imagination.

In Louisiana, a 40-acre monster

has been consuming the bayou for two years.

In Spain, sinkholes are cracking apart historic towns

and leaving others in ruins.

NARRATOR: And in Florida,

a sinkhole becomes one family's worst nightmare.

MAN: We heard my brother scream.

There was no floor, no ground, no nothing.

Everything was gone.

Everything was gone.

NARRATOR: Why is the ground under our feet collapsing?

What dangers lurk beneath the surface?

Whose home will be next?

"Sinkholes: Buried Alive," right now on NOVA.

Major funding for NOVA is provided by the following:

At Cancer Treatment Centers of America,

Supporting NOVA and promoting public understanding of science.

And the Corporation for Public Broadcasting,

and by PBS viewers like you.

operty in Seffner, Florida,

is all that remains of a tragedy

that will haunt the dreams of Floridians forever.

A home had stood on this spot for over three decades.

But that all changed on February 28, 2013.

The house had been occupied by Jeremy Bush,

his wife Rachel, and their daughter Hannah.

Rachel's sister Janell and Jeremy's brother Jeff

were also living there.

That night, Rachel and Jeremy had finished a late dinner,

were washing up, and about to go to bed.

Janell had just gone into her bedroom for the night,

and Jeremy's brother Jeff was already asleep in his room.

It had been a long day for everyone.

When we were just fixin' to lay down,

that's when it happened.

NARRATOR: The time was 11:01 p.m.

(rumbling)

It sounded like somebody came and hit the house with a truck.

(man screams)

Then we heard my brother scream.

RACHEL: We ran down the hallway.

I flicked on the light and opened up the door.

JEREMY: There was no floor, no ground, no nothing.

Everything was gone.

Everything was gone.

NARRATOR: And everything was still sinking.

But Jeremy makes an instant decision.

I automatically just jumped into the hole

and started screaming and yelling for my brother.

And all I could see was the corner of my brother's bed.

That was it.

His TV, his dresser and everything was gone.

I started digging with my hands and stuff,

trying to get to my brother

because I thought I could hear him yelling and screaming

for me to help him.

NARRATOR: Rescue teams were too late to save Jeff Bush,

but did manage to pull Jeremy from the ravenous hole.

The next day, cautious authorities

stretched a pole camera through Jeff Bush's bedroom window.

The sinkhole was more than 20 feet deep

and completely empty.

No bed, no body, just dirt.

Wary of more sinkholes,

the house and two neighboring homes were demolished.

Jeff Bush's body was never recovered.

The slab and hole mark his grave.

But ironically, his death may have prevented another.

JEREMY: Two months before my brother moved in,

it was my daughter's bedroom.

Hannah's bedroom.

NARRATOR: Sinkhole fatalities like this one are rare,

a remarkable fact

given the scores of sinkholes that occur each year.

MIKE FASANO: Florida is very lucky that more people

haven't been seriously injured from sinkholes,

where they were sleeping in their home

or apartment complex

and they did not get out in time.

NARRATOR: Although sinkholes threaten many regions of the world,

Florida is particularly vulnerable.

In 1981, one of the largest ever witnessed

ate two square blocks of Winter Park near Orlando,

including an unfortunate Porsche Dealership.

Sinkholes are usually not this big.

But why does Florida have so many?

Its placid surface offers no clue.

But underground, the answer becomes clear.

In many places, the bedrock is far from solid

and riddled with water-filled caves.

The caves are carved out of a porous rock called limestone,

and it's this water-soluble rock

that gives Florida its unique and dangerous footing.

At the surface, a quarry is one of the few places

where we can see Florida's hidden geology.

And this one near Tallahassee

has attracted Geological Survey specialist Harley Means.

This fossil clam is over 30 million years old.

And this is the fossil remains of an organism

that lived in a shallow sea.

Well, we're standing above sea level currently,

so how did this deposit get here?

How did it form?

Well, it turns out Florida has been shaped primarily

by the processes that drive sea level fluctuations.

NARRATOR: The landmass we now know as Florida

stayed submerged for millions of years.

Although ocean levels rose and fell,

periods of global freezing expanded ice caps,

froze glaciers and eventually lowered sea levels,

exposing the land.

The emerging surface became a dynamic mix of seawater,

wind-blown sediments

and the remains of countless marine organisms.

MEANS: So what happens is

these critters live, they die,

and they accumulate on the sea floor,

and then they get buried, they get compressed,

and ultimately, they can become rock.

That's what we're looking at here.

So... one of the properties that makes this rock

so susceptible to being dissolved away

is the ability for water to move through this.

And it has something to do with how well it's cemented together.

There's not a lot in the way of mud and whatnot

to impede the ability of this material

to transmit a whole bunch of ground water.

NARRATOR: Landscapes where water erodes limestone like this

and other rock types are called karst.

Rainwater becomes acidic

when it absorbs carbon dioxide in the air and ground

and over time can eat away the rock,

creating caverns, caves and sinkholes.

But how do sinkholes actually form?

Back in the quarry,

we can see soil above the layers of limestone.

When it rains, water passes through the soil

and enters the rock layer through cracks and fissures.

So here's one of these fracture features

that we've been talking about--

you can see it all the way from the top of the section here

right down through right in front of me here--

that plays such an important role

in the development of karst in Florida.

What happens is, rocks are brittle,

and over geologic time,

they can fracture like this has right here,

and then they provide a preferred pathway

for groundwater to flow.

NARRATOR: Rainwater from the surface passes through the cracks,

eroding the limestone and carving out a hollow space.

When the growing void reaches the underside of the soil layer,

dirt will begin to drop into the hole.

If the soil is loose or sandy, the surface will begin to sink,

gradually forming a bowl-shaped depression

called a subsidence sinkhole.

But if the soil contains clay, which is more cohesive,

the surface will not sag

even as the void underneath grows larger

until without warning, it suddenly collapses.

(loud crashing)

MEANS: These are called cover collapse sinkholes.

These are the ones that really do most of the damage,

not only in Florida, but all over the world.

NARRATOR: Globally, many regions contain limestone,

and virtually all are vulnerable to sinkholes.

Beside sinkholes, these areas are often characterized by caves

hollowed out over geologic time by water.

Because many of Florida's caves are still filled with water,

they are magnets for intrepid cave divers,

like this team led by Brett Hemphill.

Once we get to 160 feet, this should clear up real good.

We should be good.

NARRATOR: Today, the team will enter a cave system near Gainesville.

After suiting up,

they slip into a leafy looking pond called Mill Creek Sink.

The pond is actually a very deep sinkhole

and one of many portals

into Florida's vast and underexplored cave system.

HEMPHILL: With all the cave systems that we're aware of in Florida,

believe it or not,

the ones that we can dive and actually get into

represent less than one percent

of the caves that actually exist underground.

NARRATOR: Brett and other divers have explored this section before.

As always, they will look for cracks in the walls

big enough to swim through,

hoping they can find unexplored territory.

Along the way, they often find surprises.

HEMPHILL: Caves are time capsules.

You have to understand that because there's no rain or sun

that will erode away stuff,

in a cave, things are locked in time--

bones, fossils, just many things.

NARRATOR: Cave diving is very dangerous.

Currents and conditions can change quickly.

Any physical or equipment problem,

and there's no quick way to get to the surface.

Some divers never come back.

Although most cave systems are well below ground,

some get so close to the surface

they can create a sinkhole threat.

HEMPHILL: Divers can get a sense of what's occurring above them

in these particular areas

because as we're moving through the conduit underground,

you can see that the ceiling starts to become eroded away.

Many times when we're in the caves,

we can be at 80 feet,

and the conduit can be like a tube,

and all of a sudden, there's literally less than 20 feet

between us and the surface.

And these are areas that are very prone to sinkholes.

NARRATOR: The land around Florida's picturesque lakes

is also prone to collapse

because many lakes are themselves ancient sinkholes.

And there are regions where if you peeled away the surface,

it would look like Swiss cheese.

So building houses above these areas

makes sinkhole problems almost inevitable.

Usually, there are warning signs

the ground is sinking or shifting.

The driveway had collapsed.

The sinkhole just dropped out.

This door used to open.

It is a significant buckling.

This crack.

Truck fell in the test point hole.

(loud crash)

NARRATOR: But sinkholes can also occur with no warning signs,

especially during heavy rains and flooding

when water oversaturates the land,

or when farmers irrigate and draw too much water

from the underground aquifer.

Then disasters happen.

Guys, let's fill the tank.

NARRATOR: That is why Ming Ye and a team from Florida State University

are creating this sandbox model.

Be careful about the water content in the clay.

NARRATOR: They want to speed up the time it takes

for sinkholes to form in different soil environments.

YE: We put sands and rocks to mimic the geology,

and also we put water to mimic the hydrology in the field.

NARRATOR: Once the layers are stacked correctly,

the team adds some extras and fills the model with water

because today's test will focus on drawing too much water

from the underground aquifer.

So we are testing one tipping point,

that is, change of water level in the subsurface environment.

NARRATOR: The top two layers are sand and soil.

Below are limestone layers where a small void is forming--

the first step in the sinkhole process.

This process takes place

when a lot of water is drawn through the limestone layer

and begins to dissolve it.

You can't see water flowing in the sponge-like limestone,

but it is there, just as it is in the network of caves below.

HEMPHILL: A lot of people think of the Floridian aquifer

as this enormous void of water.

When you look at limestone, you think it's absolute,

but it is a very porous media, and water can move through it.

NARRATOR: The aquifer is Florida's main source of drinking water

and also provides water for irrigation.

In severe winters, farmers will often irrigate day and night

to keep fruit from freezing, and that's when problems emerge.

MEANS: It turns out when you start

to draw down the surface of the groundwater table,

those void spaces that might have once been filled with water

are now filled with air, and air does not provide

as much support as water does

and we can see more sinkholes occur.

NARRATOR: In 2010, massive pumping in Plant City

produced 60 sinkholes in this one East Tampa community.

Ming has patterned his model on the Plant City experience

and is drawing water out of the aquifer

to simulate the overpumping there.

As a result, a large sinkhole has developed under the surface,

just like the ones in Plant City.

That's because the soil layers and deteriorating limestone

are sliding downward, causing the surface to subside.

Then the team turns things around

and begins to add water at the top

as if a big storm followed overpumping of the aquifer.

The water pools into the void space,

triggering an almost immediate collapse.

For Ming Ye, there is one unmistakable conclusion.

YE: It would be a disaster

if you pump too much water from the limestone layer

and at the same time, you have a thunderstorm,

and that pours too much water on the ground.

NARRATOR: To predict sinkholes in various soil conditions,

the team will have to do the experiment many times.

Their results could inform future development

and water regulations.

But for many Floridians, the die is already cast.

Richard and Betty Theodore live in a modest house

in Spring Hill, Florida.

You know, this thing is really getting worse.

NARRATOR: Like many residents, they came here from the Northeast

to live out their golden years

in the warmth of the Sunshine State.

BETTY: We lived in a little town in Massachusetts

called Acushnet,

And we found a nice house and we bought it.

NARRATOR: But they didn't know they had moved into an area

locals call Sinkhole Alley.

BETTY: There were at least seven houses in this immediate area

that are sinkhole houses.

NARRATOR: And then it happened to their house.

RICHARD: One day, I went outside, and I see all different cracks

on the side of the house, and come to find out

we did have a sinkhole underneath the house.

When we got the house,

we didn't know we had a sinkhole.

Oh, send a bill.

408 has numerous problems.

NARRATOR: Former State Senator Mike Fasano

has been Florida's leading advocate for sinkhole victims.

FASANO: The homeowners in many of those homes

who happen to be seniors feel very vulnerable.

They've invested every dime they have in that home,

so if a sinkhole were to occur,

this is the last home they're going to be living in probably

until they die.

NARRATOR: The Theodores would like to sell their house

and move somewhere safer,

but they believe they will get very little money for it now,

so they're staying put

and hoping the house does the same.

BETTY: When we moved here, we expected to enjoy our retirement

and just see some sights.

I love Florida.

I just don't love sinkholes.

NARRATOR: Although Florida is extremely prone to sinkholes,

especially in the central portion of the state,

globally, it is hardly alone.

In Spain, there are communities

that have been living with sinkholes for centuries.

This is the medieval city of Calatayud.

Founded and fortified by Muslim Moors in the eighth century

and conquered by the Christian king

Alphonso the Battler in the 12th century,

today, it is a charming jewel cursed by a plague of sinkholes.

FRANCISCO GUTIERREZ: A peculiar aspect of this city

is that most the buildings have been built on gypsum,

and this is a soluble rock,

and that's the reason why sinkhole problems

are quite common.

NARRATOR: Gypsum surrounds and underpins this city.

As another form of karst, it is easily dissolved by water,

but is more porous than limestone

and dissolves even faster.

So the city's buildings are constantly cracking.

GUTIERREZ: Problems affect the daily life of the people.

NARRATOR: Several of the city's most beautiful buildings

are endangered.

But structural fixes here are difficult and very expensive,

so damage everywhere keeps reoccurring.

GUTIERREZ: Cracks are very common,

and obviously people try to repair them with plaster,

but obviously that does not solve the problem.

It's just a makeup.

It improves the aesthetics.

(church bell ringing)

NARRATOR: In an effort to save a leaning church tower,

engineers lopped off the top,

hoping the surgery would keep it from crashing down.

The storks don't seem too concerned.

Like the people of Calatayud,

they are simply making the best of their tilted world.

GUTIERREZ: The development of the sinkholes will keep on occurring,

but people here get used to that problem.

They can obviously cope with it.

NARRATOR: Although Calatayud has managed to survive its unstable ground

for centuries,

other towns in the region have not been as fortunate.

This is an aerial photograph of Puilatos,

a sparkling new town built in the 1950s

to provide better housing for local farmers

and their families.

Today, it's just piles of rubble,

the victim of a sinkhole-laced terrain.

(translated): It was a neighborhood of houses.

A street went through here.

This is all very changed now.

NARRATOR: Javier Rome was one of the town's original inhabitants.

ROME (translated): Here, we still see a manhole cover that was built in 1962.

This was the end of the town.

The water ran around the perimeter.

We came in 1956.

The problems with the cracks and the sinking

started a few years later.

NARRATOR: Like most people in Puilatos,

Javier came from a family of farmers.

In fact, he still is one.

Unlike his ancestors, he tills the fields

with the machines of modern agriculture.

But every generation has worked on land

where layers of porous rock lie just beneath the soil.

And when farmers irrigate, the water pours onto the surface

and then down to mineral deposits

that here are composed primarily of salt.

In geologic terms, salt is an evaporite

and, like gypsum, can dissolve very easily.

One of the characteristics of the evaporative areas

is that the dissolution processes may be very fast

and the probability of occurrence of sinkholes

may be very high.

NARRATOR: Just walking the fields in places like these

can be extremely dangerous.

Experienced farmers used to wear long wooden sticks

tied to their shoulders.

The sticks had nothing to do with tending crops.

They had a far more important function...

(gasps)

...keeping farmers from entirely disappearing into sinkholes.

(grunting)

MIGUEL LIZAGA (translated): When I was a kid, I knew some elders who carried the sticks

because they made them feel safer.

I know people who fell in one and got hurt,

and also machinery.

When the sinkhole was big,

whole harvesters have fallen inside.

NARRATOR: By the 1970s, there were so many sinkholes in Puilatos,

the government condemned the once model community

and tore it down.

ROME (translated): We lived here for 19 years.

It was not very pleasant to leave.

We were nostalgic for what we were leaving behind.

NARRATOR: The sinkholes that destroyed Puilatos

did not disappear with the village.

Today, in the shadow of the destroyed town,

a growing, water-filled sinkhole threatens a major highway.

A team from the University of Zaragoza

is using Ground Penetrating Radar

to assess the effect of the sinkhole on the highway.

GPR uses radio waves

to create a kind of sonogram of conditions under the ground.

GUTIERREZ: Here, we have a very large sinkhole.

This sinkhole is more than 200 meters across.

We can see that the ground and also the overlying highway

is subsiding by sagging.

And this is the kind of sinkhole

that's very difficult to remediate

because it's related to the dissolution of salt at depth,

and that's something you cannot stop.

NARRATOR: As in Florida, sinkholes in Spain are a constant threat

and not just to roads and houses.

Just before its famous high-speed train was to make

an inaugural run, a sinkhole was discovered under the track.

With speeds approaching 180 miles per hour,

any deviation in the track alignment

could cause a derailment and bring results

similar to this 2013 operator-caused catastrophe.

Globally, as more sinkholes emerge near infrastructure,

housing and other forms of development, one thing is clear.

GUTIERREZ: Sinkholes can be accelerated

or even triggered by various human activities.

NARRATOR: Poorly designed and overloaded sewage systems helped create

this enormous sinkhole in the center of Guatemala City.

Damming the Jordan River

has made the desert edge of the Dead Sea

look like the surface of the moon.

And melting permafrost from global warming

could be responsible for spectacular holes

in far Siberia.

20% of the earth's land surface contains the soluble rocks

of karst terrain.

This means there are millions of acres in scores of countries

where human activities can increase the risk of sinkholes.

That is why Francisco Gutierrez studies these areas

so intensively.

These rare limestone monsters are extremely old,

probably forming long before any human activity.

But as it turns out, we can create our own sinkhole monsters

and in a mere fraction of that time.

There's no better example of human-created sinkholes

than in southern Louisiana's Bayou Corne.

Bayous are home to marshes and slow moving rivers,

slender-rooted cypress trees,

birds, fish, insects and, of course, alligators.

Here, the natural world is interlaced with metal roots:

giant pipelines transporting gas, oil and salty brine

to the region's many petrochemical plants.

PATRICK COURREGES: One of the keystones of our economy has always been

development of the natural resources.

NARRATOR: Patrick Courreges works

with the Louisiana Department of Natural Resources.

Louisiana has always been rich in oil and natural gas,

which are very critical to almost everything we do.

NARRATOR: Bayou people, like longtime resident Dennis Landry,

are not too concerned about all the oil and gas operations,

except when they see bubbles like these.

And bubbles in this very spot

began a saga few here will ever forget.

LANDRY: My wife Pat and I were just taking

a leisurely boat ride

and when we got close to the pipelines here,

I saw some unusual bubbling in the water.

So I looked up and I could see a pipeline sign,

so I immediately thought,

"Well, surely it's got to be the pipeline leaking."

NARRATOR: Pipelines from natural gas wells

crisscross the undersurface of the bayou.

Natural gas is odorless, highly flammable

and explosively dangerous...

so reports of unusual bubbling go directly to John Boudreaux,

the Assumption Parish director of emergency management.

BOUDREAUX: We received a call about bubbling

that was occurring in Bayou Corne,

and yes, there was bubbling

and it was right near the pipeline crossing.

NARRATOR: But Boudreaux had to eliminate

the possibility it was swamp gas, which results

from rotting organic matter and is not a threat.

So, like a forensic detective,

he sped to the site to take the gas's fingerprints.

Boudreaux had to catch the bubbles with a funnel

and feed the data to a methane detector

to see if it was natural gas from a leaking pipeline

or just swamp gas.

BOUDREAUX: The methane detector indicated it was not swamp gas--

that it was a natural gas from a deep source.

NARRATOR: But almost immediately, more bubbles began to appear

in too many places to originate from a single leaking pipe.

Then they had bubbles coming up in the neighborhood.

It's not just in the bayou.

They were coming up right down the road over here,

bubbling up like Alka-Seltzer.

NARRATOR: And then the swamp began to tremble with earthquakes.

Very unusual to have earthquakes here in south Louisiana.

NARRATOR: As the earthquake activity increased,

the community began getting very nervous.

COURREGES: We have natural gas bubbling, but the pipelines are okay.

We're feeling tremors.

It's kind of a mystery because the clues don't really add up.

NARRATOR: Then in the predawn hours of August 3, 2012...

the bayou cracked open in a giant circle

as if the land had been struck by a colossal meteor.

LANDRY: There was a huge hole in the middle of the swamp,

at least a couple of football fields.

BOUDREAUX: I have never seen trees

that have turned over

and the root system is now pointing up.

LANDRY: I could smell a strong smell

of diesel or petroleum or something.

BOUDREAUX: I'd never seen anything like it.

NARRATOR: What was once a shallow, tree-filled swamp became a lake,

covered with an oil slick.

(tree trunk cracking)

Boudreaux used an amphibious vehicle to reach the middle

so he could measure the depth of the sinkhole that created it.

BOUDREAUX: That initial depth

that I checked was 384 feet.

NARRATOR: The hole was almost 40 stories deep.

What could possibly cause this to happen?

The immediate suspect was fairly close to the sinkhole

and lay under this clearing:

the mile deep, and previously abandoned,

Texas Brine salt cavern.

There are several caverns in Bayou Corne

because it sits atop an enormous geologic formation

called a salt dome.

Salt domes are ancient salty sea beds that rise up like pillars

when downward pressure from accumulating sediment

forces the buoyant salt upward.

They are extremely valuable

because oil and gas can get trapped on their perimeters.

And of course, there's the salt.

Mined for centuries to help preserve food

and enhance its flavor,

today salt is also processed into chlorine gas,

a major component for making plastic, bleach and paper.

But traditional mining is expensive,

so companies like Texas Brine brine it.

Water at high pressure is sent down a pipe.

It easily dissolves away the salt and creates a watery cavern

that grows larger as brining increases.

The salty brine is then pumped to the surface, stored in tanks

and piped to chemical plants.

COURREGES: It's a mechanically simple way to get it and deliver it

to flush fresh water down, bring salt water up

and ship that salt water up for further processing.

NARRATOR: Sinkholes on salt domes have happened before.

Too much brining in Daisetta, Texas,

caused the top of this dome to crack apart

and the surface above to subside into a large sinkhole.

The Bayou Corne sinkhole was far deeper than Daisetta

and its origin mysterious.

Although Texas Brine's salt cavern

was not directly under the sinkhole,

the state ordered the company to inspect the cavern

to see if it triggered the collapse.

They sent down a drill that encountered no problems

for the first 1,000 feet, but then struck rock and sediment.

This material had to come from outside the salt cavern--

meaning the company had brined

too close to the edge of the dome, cracking it open.

Soil and rock had cascaded through the breach,

filling the deep cavern,

and this in turn sucked down the surface in a massive sinkhole.

COURREGES: What we know now is

the bubbling appearing over a widespread area,

the tremors people were feeling,

those were the indicator that that cavern had begun

or was well into the process of collapsing.

NARRATOR: With several lawsuits pending,

Texas Brine refused to talk to us on camera.

The company does accept responsibility for the sinkhole,

but denies their negligence caused the accident.

Using independently created three-dimensional measurements,

the company points out that the wall of the salt dome

curved farther inward than anyone realized.

So as their brine cavern expanded,

they couldn't have known it had gotten too close to the edge.

Although the legal process is ongoing,

for months, Bayou Corne was a hellish place.

Rising waters quickly overwhelmed a protective berm,

and gas shot out of the vent wells.

John Boudreaux patrolled the site every day

because his instincts told him

the sinkhole trouble was far from over.

Then, almost a year after it all began, he got a call

from the seismic monitoring station.

He says, "Something's happening at the sinkhole.

I can see it on our equipment."

NARRATOR: Boudreaux arrived at the edge of the sinkhole just in time.

BOUDREAUX: I saw the bubbling in the tree areas

and it looked like those trees were sinking, and they were.

Pulled out my iPhone.

MAN: They're moving, John.

BOUDREAUX: Yep, I got it.

I got it.

NARRATOR: The video that John Boudreaux shot that day

made Bayou Corne a national news story and a YouTube hit.

Despite the real and present danger,

some residents never left their bayou homes.

Dennis Landry has stayed throughout

in the community he founded and calls Sportsman's Paradise.

LANDRY: It was a dre

to develop a beautiful waterfront community

here in beautiful Bayou Corne.

NARRATOR: He believes residents are reasonably safe.

LANDRY: We were assured

that the hole would be monitored 24 hours a day.

They started putting in vent wells to vent the gas,

to get the gas out of the aquifer.

They came in, they put air monitors in our homes,

and like that they gave us some sense of security

that if gas would get into our homes we would know about it

in plenty of time to get out.

NARRATOR: There's still a lot of gas, with one of the biggest pockets

right under the emergency command center.

But recently things have started to calm down.

The sinkhole appears to be plugging up.

The gas flares are less intense.

And most of the oil is gone.

Unfortunately, so too are the people,

especially in the older community across the highway

from Sportsman's Paradise.

Over the past two years,

Texas Brine bought out homeowners afraid to return

and settled a class action lawsuit from others

angry their homes were now worthless.

But it's little solace

for the people who once called this home.

MIKE SCHAFF: My friends over here, Janet and Jerry,

lived here for 30 years.

They were bought out in the first round by Texas Brine.

NARRATOR: Mike Schaff is one of the last people leaving

this once lively community now going to seed.

SCHAFF: We didn't have much choice.

Pretty much everybody decided to leave.

Bayou Corne is not going to be what it was before.

It's never going to be the vibrant community it was.

The other day I had to decide whether or not to save my house,

to stay here or not,

or turn it over to Texas Brine so they could tear it down.

It felt, to be honest with you, it fellike turning her over

to... an innocent person over to the executioner.

NARRATOR: With most of the houses abandoned,

is the saga of the Bayou Corne sinkhole over?

Will a vibrant community ever return?

Will there be long-term health impacts to the bayou

and the people who lived here?

As Louisiana considers new brining regulations

to guard against future mishaps,

there is a broader question to answer:

What can be done right now to protect people everywhere

from the worst consequences of sinkholes?

Back in Florida, it's hard to imagine sinkholes forming

under these lovely homes.

But they have.

Jimmy and D'Etta Ferraro and D'Etta's mother Midge

have lived here for four years,

the last two with confirmed sinkhole activity that began

with cracking walls and a subsiding pool deck.

Go! We're good!

NARRATOR: To stabilize and strengthen the soil,

the Ferraros' insurance company wants to pump grout,

a cementlike mixture, under the foundation.

It's a technique that has been used successfully

all over the state.

But under certain conditions,

pouring grout has actually caused huge,

terrifying sinkholes, like this one in Dunedin, Florida.

The Ferraros want their house pinned with steel,

an expense the insurance company is resisting.

Our home is the most expensive investment that we make

and now I have a home with a sinkhole,

and I have to worry about my family's safety all the time.

NARRATOR: The Ferraros have decided to bring in a geotechnical team

to analyze the problem.

While the equipment is being set up,

lead engineer Byron Anderson begins with a simple test.

ANDERSON: One of the quick ways I can determine

whether or not there's a problem at a property

is by using a fiberglass probe

to just stick it down into the ground.

And what you would normally expect to see is that the rod

would just go a few inches deep and you would get to refusal.

But at this property what I can find

is there are areas in which the rod just almost free-falls

down into the ground.

And these areas are little depressions

that are likely related to sinkhole activity.

And one of the old wives' tales that we have often thought of

whenever we're doing sinkhole investigations

is that the water is being dropped out of the grass,

going into a sinkhole, causing the vegetation to die.

As I was doing earlier, I can just take this probe

and in the center of what is this void,

you can see that it just very easily goes down into the earth.

NARRATOR: There are obvious problems here,

but how extensive and dangerous are they?

A drilling team is gathering soil samples

on the other side of the house.

The men will pull up samples all the way down to bedrock.

If they find loose, sandy soil at depth,

this could indicate that upper layers of soil

are dropping down a sinkhole.

BRIAN JORY: Okay, that was a sample of sand from a depth of 20 feet.

And that's very typical of upper soils

down to about 30 feet in Florida.

NARRATOR: It could also be typical of a growing sinkhole.

The team is creating a Ground Penetrating Radar map

of the entire property

in order to reveal areas of displaced soil

and detect the extent of any empty spaces.

From this and other tests, they create a preliminary sketch

that indicates not one, but three potential sinkholes

on two sides of the house.

And they can't rule out the possibility

that there's one giant sinkhole under the entire property.

The results do not surprise Byron Anderson.

ANDERSON: With this property what concerns me

is that we have this retention pond area that's off to the left

and we also have a wetland area that's north of here.

This retention area and wetland area are old sinkholes

that have formed in the geologic past.

This is another example

of why you should not have built houses on this property.

NARRATOR: Unfortunately, houses aren't the only structures

Floridians should be worried about.

Jacki Clark is dodging traffic

along a highway near Gainesville.

Somewhere underneath her and the traffic,

Brett Hemphill and his dive team are in the Alachua Cave system

setting up transponders.

On the surface, Jacki, a diver herself,

uses a disc-like receiver to detect their signals

and locate their precise position.

CLARK: What we are trying to do is we're trying to figure out

where Alachua Sink cave passage actually goes.

So I am using this radio locate of where it's going

and where in relation to all these buildings

the cave passage runs.

NARRATOR: As it turns out,

the divers are indeed swimming underneath this major highway

and the cave system could go

under several business establishments.

We've blurred the names

because owners and customers could get nervous

at the thought that watery caves might be under their feet.

Each location point becomes another step

for mapping the route of the divers

and this particular cave system.

It looks like the system misses nearby businesses.

But the divers discovered a branch

that heads straight for a big box store

and goes right underneath it.

BRETT HEMPHILL: There have been several times

when we've been exploring cave systems in Florida

where we actually realize after we create a map

or we do a radio location

that the conduit indeed runs under a house

or an actual commercial property.

First thing they'll ask you is,

"Is there a chance that my house could fall into a sinkhole?"

NARRATOR: Although some cave systems are close to the surface

and can pose a threat, most are well below ground

and, like this one, are not a cause for concern.

But in Florida, many people do live on dangerous ground

and are worried about their property and personal safety.

JIMMY FERRARO: All we do is think about,

"Is the house going to collapse today?

"When is that going to happen?

"Is it going to collapse while we're sleeping,

like the house in Seffner, Florida?"

It's a constant weight on our shoulders.

NARRATOR: Prospective homeowners can check public records to see

if a given neighborhood is prone to sinkholes before moving in.

And the state is compiling a comprehensive sinkhole map

showing the most active regions.

There is also technology that can detect sinkhole activity

before houses are even built.

So why are developers building in dangerous areas?

And why are people moving into them?

If you're a land owner and you own a tract of land

that, let's say, is 50 acres,

well, you don't want to test that land

because once you find sinkholes,

your land isn't worth anything anymore.

NARRATOR: And if the land gets sold?

ANDERSON: If you're the developer and you find sinkhole activity,

then all the money that you've sunk

into the development is lost.

NARRATOR: What about local government protection?

ANDERSON: If the county finds that there is sinkhole activity

on that 50-acre tract,

they will never get the tax revenue associated with it.

NARRATOR: And the result?

Left the homeowners to bear the burden of dealing

with these sinkhole conditions.

NARRATOR: But perhaps the biggest problem is Florida's booming population.

As people flock to the Sunshine State, more houses, roads

and businesses will surely bring more sinkholes

and perhaps more fatalities as well.

So the efforts of researchers and geologists

to better understand the precise conditions

for sinkhole formation

and the tipping points for their collapse

may one day give us the data to help save lives.

The only question is, "Will we use it?"

This NOVA program is available on DVD.

To order, visit shopPBS.org, or call 1-800-PLAY-PBS.

NOVA is also available for download on iTunes.