Science & Environment

Sinkholes: A deadly threat from Florida's 'underworld'

Collapse near Disney World Image copyright Getty Images
Image caption Guests left just in time before this resort building near Disney World slumped into a 20m-wide hole last year

"It's like this thing was alive…it was churning, moving around…making noises, you know…like a growl."

It's an image that still haunts police officer Deputy Douglas Duvall who, on the evening of 28 February 2013, responded to an emergency call in the suburban calm of Tampa, and found himself face to face with the Florida underworld.

Inside a detached bungalow, the ground had opened and swallowed the sleeping body and the bed of 37 year-old Jeff Bush.

His brother Jeremy was frantically trying to dig him out, but Jeff's body was sucked into the depths and never found.

Only the efforts of first responder Douglas Duvall hauling Jeremy out of the churning pit prevented a second tragedy.

The natural trapdoor that opened up and claimed the life of Jeff Bush is called a "sinkhole". It is far from the only case.

In the last few years, vast sinkholes have appeared overnight from as far afield as China and Guatemala, but it's Florida where the fear is greatest.

Just last August, a resort complex near Disney World collapsed into a huge 20m hole.

It was to investigate this devastating phenomenon that I travelled to Florida to try to understand what caused the sinkhole that killed Jeff, and why the geology of this state makes it the sinkhole capital of the world.

It's possible to explore some of these natural shafts and descend within the voids beneath, at places like Ladder Cave in Citrus County.

Here you can see how acid-tinged rain and ground water slowly eats away at limestone bedrock below, producing cavities in the subsurface.

Image copyright Getty Images
Image caption There was initially little external sign of the sinkhole that had swallowed up Jeff Bush
Image copyright AP
Image caption Only after demolition crews moved in to raze the house was the deep depression exposed

Often, surface sand and mud gets washed into these to fashion a pockmarked landscape of pits and depressions which we call karst.

The trouble is, sometimes, the subterranean world of caves and caverns break through the surface cover to drag down whatever lies above.

These "cover-collapse" sinkholes are the deadly threat that lurks in the Florida underworld.

Investigations revealed that a cover-collapse sinkhole had lain directly beneath Jeff Bush's bedroom.

Bill Bracken, the structural engineer who worked with the emergency workers at Jeff's house, showed me the footage that he took from within the hole that fateful night.

It makes chilling viewing. When it opened and soil began to fall inward, a suction force was exerted on the concrete floor above, eventually wrenching it down along with everything in that room.

Sinkhole collapses are pretty commonplace across Florida. Virtually the whole of the Sunshine state, from the Keys in the south to the border with Georgia in the north, is a vast limestone platform that is flushed with groundwater below and has a humid climate that rains down plenty from above.

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Media captionHow limestone sinkholes are formed

That water keeps the lawns green, fills the swimming pools and provides drinking water for millions. But the waters are also consuming Florida's soluble limestone foundations.

The result is a state collapsing in on itself. Amid the city streets, quiet suburbs and citrus groves, holes are often opening up to reveal a new hidden Florida.

Residents are, understandably, nervous about the Florida concealed beneath. As soon as any cracks appear in their buildings, nervous homeowners call in geotechnical experts.

Over 6,500 sinkhole insurance claims are reported each year. All of which makes Florida's sinkholes a boom for lawyers and geologists.

Image copyright AP
Image caption Like a scene in a disaster movie, this vast cavity opened at a street intersection in Guatemala City in 2010
Image copyright AFP
Image caption Here, policemen observe another huge hole that formed in Guatemala in 2007
Image copyright Science Photo Library
Image caption Some sinkholes, such as Mexico's Gran Cenote, open into spectacular subterranean landscapes
Image copyright AFP
Image caption This collapse in Beijing swallowed up a truck. Sinkholes in China are often blamed on construction work

What is not commonplace are sinkhole tragedies. Jeff Bush's death is Florida's first fatal sinkhole collapse in decades.

His suburban district of Seffner lies in a sinkhole "sweetspot" - a cluster of collapses pepper the west central part of the state around the city of Tampa.

Image caption Iain Stewart went in search of the reasons for Florida's first fatal sinkhole collapse in decades

His death has created unease among the sedate, retiring gated communities of west Florida. Because the sinkhole scourge is on the rise.

Quite why sinkholes are becoming ever more prominent in Florida is uncertain. Their triggers are enigmatic, though the fact that there is a "sinkhole season" suggests that Florida's climate has something to do with it.

In the summer months, the Gulf Coast's hurricanes deluge the state, dumping tonnes of water on the land over a matter of hours, weighing down the soil and collapsing the roofs of caves below.

In the dry season, drought conditions can lower the water table, reducing pressure in water-filled voids and causing their unsupported sides to implode.

Dramatic changes to the Floridian water table can also come from another more surprising source. The state's warm, wet weather and fertile soil cover has made it ideal for agriculture, and makes it, alongside California, the fruit basket of America.

Its famous citrus groves and fruit fields are irrigated in part from groundwater drawn from Florida's underground aquifer.

Unlike the year-round warmth of California, the winter months in Florida can be cold and its fruit, particularly its vast strawberry crop, is prone to frost damage.

Image copyright Getty Images
Image caption This sinkhole in Dunedin, Florida, pulled down the rear portion of a house
Image copyright Rex Features
Image caption This aerial shot shows the extent of damage to the resort building in Clermont, near Disney World

So when sudden cold snaps strike, farmers respond with an intense spraying of warm groundwater onto the strawberry fields.

This aggressive groundwater pumping, however, can drop aquifer levels by tens of metres overnight. When this has happened in the past, large numbers of sinkhole collapses have occurred.

There is no evidence that groundwater pumping for agriculture was the trigger for last year's lethal collapse in Seffner.

However, it is a reminder of the growing human pressures that are being placed on Florida's natural support system.

The lure of the Florida sun is drawing ever more people to the state, and our urban sprawl is advancing into wild land primed with lethal sinkhole traps. In the past, they would have gone unnoticed. But not now.

So, in a way, the real reason for Florida's growing toll of sinkhole damage is ourselves.

Horizon: Swallowed By A Sinkhole is on BBC2 at 9pm on Monday 3 February

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