Sinkholes—Buried Alive

The catastrophic collapse of the ground beneath our feet is a growing worldwide hazard. Airing October 26, 2016 at 9 pm on PBS Aired October 26, 2016 on PBS

  • Originally aired 01.28.15

Program Description

In a leafy suburb near Tampa, Florida, on February 28, 2013, a giant hole opened up under the bedroom floor of Jeffrey Bush, swallowing him as he slept. His body was never found. Bush was a victim of a sinkhole—a worldwide hazard that lurks wherever limestone and other water-soluble rocks underpin the soil. When carbon dioxide from the air dissolves in rainwater, it forms a weak acid that attacks soft rocks, riddling them with holes like Swiss cheese. Sinkholes can occur gradually when the surface subsides into bowl shaped depressions or suddenly when the ground gives way. These geological hazards have swallowed highways, apartment buildings, horses, camels, even golfers, with monster-size holes cracking the earth from Siberia to Louisiana. Filled with compelling eyewitness video of dramatic collapses, and following scientists as they explore the underlying forces behind these natural disasters, NOVA travels the globe to investigate what it's like to have your world vanish beneath your feet.

Transcript

Sinkholes—Buried Alive

PBS Airdate: January 28, 2015

NARRATOR:They can occur suddenly and catastrophically. They have swallowed cars, animals and people. They have destroyed homes…

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.

RACHEL BUSH (911 caller): The floor just collapsed, and my brother-in-law is in there.

NARRATOR: And in Florida, a sinkhole becomes one family's worst nightmare.

JEREMY BUSH (Sinkhole survivor): We heard my brother scream. There was no floor, no ground, no nothing. Everything was gone.

RACHEL BUSH (Sinkhole survivor): 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.

This fenced off property 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 Hanna. Rachel's sister Janell and Jeremy's brother Jeff also lived 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 bedroom. It had been a long day for everyone.

RACHEL BUSH: When we were just fixing to lay down, that's when it happened.

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

JANELL WHEELER (Sinkhole survivor): It sounded like somebody came and hit the house with a truck.

JEREMY BUSH: Then we heard my brother scream.

RACHEL BUSH: We ran down the hallway. I flicked on the light, and opened up the door.

JEREMY BUSH: There was no floor, no ground, no nothing. Everything was gone.

RACHEL BUSH: Everything was gone.

NARRATOR: And everything was still sinking, but Jeremy makes an instant decision.

JEREMY BUSH: 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 T.V., his dresser and everything was gone.

I started digging with my hands and stuff, trying to get to my brother, 'cause I thought I could hear him yelling and screaming for me to help him.

RACHEL BUSH (911 caller): The house just fell through. The bedroom floor just collapsed and my brother-in-law is in there. He's underneath the house.

NARRATOR: Rescue teams were too late to save Jeff Bush but did pull Jeremy from the ravenous hole.

The next day, cautious authorities stretched a pole camera through Jeff Bush's bedroom window. The sinkhole was over 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 BUSH: 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 (Former Florida State Senator): 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.

From 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.

G.H. “HARLEY” MEANS (Geologist): 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.

HARLEY MEANS: So what happens is these critters live, they die and they accumulate on the seafloor, 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 what not to impede the ability of this material to transmit a whole bunch of groundwater.

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.

HARLEY MEANS: So here's one of those 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.

HARLEY 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.

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.

BRETT HEMPHILL (Cave Diver): 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 will find unexplored territory. Along the way, they often find surprises.

BRETT 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.

BRETT HEMPHILL: Divers can get a sense of what's occurring above them, in these particular areas, because, as we move into the conduit underground, you can see that the ceiling starts to come eroded away.

Many times, when we're in the caves, we can be at 80 feet, and the conduit will be like, be like a tube. And all of a sudden, we'll look up… there's literally less than, you know, 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.

DAN FRITZ: The driveway had collapsed.

PAM: The sinkhole just dropped out.

JIMMY: This door used to open.

BOB DONAHUE: There's a significant buckling.

D'ETTA FERRARO: This crack, up…

PAM: Truck fell in the test point hole.

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.

That is why Ming Ye and a team from Florida State University are creating this sandbox model. They want to speed up the time it takes for sinkholes to form in different soil environments.

MING YE (Florida State University?): 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.

MING YE: 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.

BRETT 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.

HARLEY 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 and…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 over-pumping 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 over-pumping of the aquifer. The water pools into the void space, triggering an almost immediate collapse.

For Ming Ye, there is one unmistakable conclusion.

MING 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 pour 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. Like many residents, they came here from the northeast, to live out their golden years in the warmth of the “Sunshine State.”

BETTY THEODORE (Florida Home Owner): We lived in a little town in Massachusetts, called Acushnet. And we found a nice house and bought it.

NARRATOR: But they didn't know they had moved into an area locals call “Sinkhole Alley.”

BETTY THEODORE: There were, there was at least seven houses in this immediate area that are sinkhole houses.

NARRATOR: And then it happened to their house.

RICHARD THEODORE (Florida Home Owner): One day, I went outside to 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.

NARRATOR: Former State Senator Mike Fasano has been Florida's leading advocate for sinkhole victims.

MIKE FASANO: The homeowners in many of those homes, who happen to be seniors and 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, 'til 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 THEODORE: When we moved, here we expected to enjoy our retirement and just see some sights. I love Florida, I just don't like 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 Alfonso the Battler in the twelfth century, today, it is a charming jewel, cursed by a plague of sinkholes.

FRANCISCO GUTIÉRREZ (Geologist): A peculiar aspect of this city is that most of the buildings are built on gypsum rock, which is a very 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 and sinking, from sinkhole activity underneath them.

Several of the city's most beautiful buildings are endangered, but structural fixes here are difficult and very expensive. So damage everywhere keeps reoccurring.

FRANCISCO GUTIÉRREZ: Cracks are very common. People try to repair them with plaster, but, obviously, that does not solve the problem. It's just a makeup. It improves the aesthetics.

NARRATOR: In an effort to save a leaning church tower, engineers lopped off the top, hoping the surgery will 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.

FRANCISCO GUTIÉRREZ: 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.

JAVIER ROME (Original Inhabitant of Puilatos, Spain): 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.

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.

JAVIER ROME: 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.

FRANCISCO GUTIÉRREZ: One of the characteristics of the evaporative areas is that the dissolution processes may be very fast, and the probability of occurrence of sinkholes in those areas 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: keeping farmers from entirely disappearing into sinkholes.

MIGUEL ANGEL LIZAGA PUEYO (Original Inhabitant of Puilatos, Spain): When I was a kid, I knew some elders who carried the sticks, because they made them feel safer. Yes, 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.

JAVIER ROME: 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 Zaragosa is using ground-penetrating radar to assess the effect of this sinkhole on the highway. G.P.R. uses radio waves to create a kind of sonogram of conditions under the ground.

FRANCISCO GUTIÉRREZ: Here we have a very large sinkhole. This sinkhole is more than 200 meters across. We can see the ground, and over there, 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.

FRANCISCO GUTIÉRREZ: 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.

Twenty percent 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 (Lousiana Department of Natural Resources): One of the keystones of our economy has always been development of natural resources.

NARRATOR: Patrick Courreges works with the Louisiana Department of Natural Resources.

PATRICK COURREGES: 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.

DENNIS LANDRY (Resident of Bayou Corne): My wife, Pat, and I were just taking a leisurely boat ride, and when we got close to the pipeline sign, 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.

JOHN BOUDREAUX (Assumption Parish, Louisiana Director of Emergency Preparedness): 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.

JOHN 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.

JOHN BOUDREAUX: 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.

JOHN BOUDREAUX: Very unusual to have earthquakes here in South Louisiana.

NARRATOR: As the earthquake activity increased, the community began getting very nervous.

PATRICK COURREGES: We have natural gas bubbling, but the pipelines are okay. We're feeling tremors. It's kind of a mystery, 'cause 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.

DENNIS LANDRY: There was a huge hole in the middle of the swamp; at least a couple of football fields.

JOHN BOUDREAUX: I have never seen trees that have turned over and the root system is now pointing up.

DENNIS LANDRY: I could smell a strong smell of diesel or petroleum or something.

JOHN BOUDREAUX: I've never seen anything like it.

NARRATOR: What was once a shallow, tree-filled swamp became a lake, covered with an oil slick.

Boudreaux used an amphibious vehicle to reach the middle, so he could measure the depth of the sinkhole that created it.

JOHN 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 scores of salt caverns under 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 around their edges 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.

PATRICK COURREGES: It's a mechanically simple way to get it and deliver it, to flush water down, bring saltwater back up and ship the saltwater 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 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.

PATRICK 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.

JOHN BOUDREAUX: 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.

JOHN BOUDREAUX: I saw bubbling in the tree areas. It looked like trees were sinking, and they were. And pulled out the, the, my iPhone.

NARRATOR: The video that John Boudreaux shot that day made Bayou Corne a national news story and 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.

DENNIS LANDRY: It was a dream of mine to develop a beautiful waterfront community here in beautiful Bayou Corne.

NARRATOR: He believes residents are reasonably safe.

DENNIS 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. Now, to protect those residents who were staying, they came in, they put air monitors, and, like that, they'd get 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 (Former Resident of Bayou Corne): My friends 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.

MIKE SCHAFF: We didn't have much choice. Pretty much everybody decided to leave. Bayou Corne isn't 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 to save my house, to stay or not, or turn it over to Texas Brine so they can tear it down. It felt…to be honest with you, it felt like turning over an innocent person 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 Ferarro 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.

To stabilize and strengthen the soil the Ferarro's insurance company wants to pump grout, a cement-like 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 Ferrero's want their house pinned with steel, an expense the insurance company is resisting.

JIMMY FERRARO: 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 Ferraro's 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.

BYRON ANDERSON (Engineer): One of the quick ways I can determine whether or not there's a problem at the 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 the refusal. But on this property, there are areas where the rod somewhat freefalls down into the ground. And these areas are depressions that are likely related to sinkhole activity.

One of the old wives tales that we have often thought of, whenever we do 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 it 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.

BYRON ANDERSON: Okay, that was a sample of sand from a depth of 20 feet. 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.

BYRON ANDERSON: With this property, what concerns me is that we have this retention pond area, off 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 have never 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 disk-like receiver to detect their signals and locate their precise position.

JACKI CLARK (Cave Diver): What we were trying to do is figure out where the Alachua Sink cave passage actually goes. So I am using this radio locator to see 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 where, 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 commercial property. First thing they'll ask you is, “Is there a chance that my house could it 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, 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, detailing 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?

BYRON ANDERSON: If you're a landowner, and you are on a tract of land, and let's say it's 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?

BYRON 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?

BYRON 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?

BYRON ANDERSON: They've 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?

Broadcast Credits

WRITTEN, PRODUCED AND DIRECTED BY
Larry Klein
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A NOVA production by Lawrence Klein Productions, LLC for WGBH Boston in association with ARTE France.

© 2015 WGBH Educational Foundation

All rights reserved

This program was produced by WGBH, which is solely responsible for its content.

IMAGE:

Image credit: (Sinkhole in Guatemala)
© DANIEL LECLAIR/X00162/Reuters/Corbis

Participants

Byron Anderson
Engineer
John Boudreaux
Director of Emergency Preparedness
Jeremy Bush
Rachel Bush
Jacki Clark
Patrick Courreges
La. Dept. of Natural Resources
Mike Fasano
Former Florida State Senator
James Ferraro
Francisco Gutiérrez
Geologist
Brett Hemphill
Brian Jory
Engineer
Dennis Landry
G. H. "Harley" Means
Geologist
Miguel Angel Lizaga Pueyo
Javier Rome
Mike Schaff
Richard and Betty Theodore
Janell Wheeler

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