What Causes Sinkholes?

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Author: Jon Henley / Video

Jeremy Bush, Seffner, Florida

Last week, in a quiet residential suburb east of Tampa, Florida, the Earth opened up and swallowed a man. Jeff Bush, 37, was tucked up in bed late on Thursday evening when his entire bedroom floor simply gave way with a deafening crash that his brother, in the room next door, later described as “like a truck hitting the house”.

Jeremy Bush, 35, heard his brother’s scream and rushed towards his bedroom. “Everything was gone,” he told local television stations. “My brother’s bed, my brother’s dresser, my brother’s TV. My brother was gone. All I could see was the top of his bed, so I jumped in and tried digging him out. I thought I could hear him screaming for me and hollering for me.”

As the house’s floor threatened to collapse further into a gaping hole more than 9m across and 15m deep, a sheriff’s deputy who had arrived on the scene with the emergency services eventually pulled Jeremy to safety. Jeff remained trapped. “I couldn’t get him out,” Jeremy said. “I tried so hard. I tried everything I could. No one could do anything.”

As Jeremy and four others, including a two-year-old child, were led away uninjured, rescue teams lowered a microphone and video camera into the hole, but it was soon apparent that Bush could not have survived. By Saturday, the search for his body had also been abandoned. “We just have not been able to locate Mr Bush, and so for that reason, the rescue effort is being discontinued,” a local official, Mike Merrill, said. “At this point, it’s really not possible to recover the body.”

When the ground begins opening up beneath our feet and plunging unsuspecting mortals into the abyss, some may be tempted to reach for the Bible and start predicting the End of Times (and a quick online search reveals that several of the wackier sort of website have not hesitated to do just that). But biblical as the story sounds, the sinkhole – as the phenomenon is called – that caused Jeff Bush’s death was not an act of God but of geology.

Natural sinkholes – as opposed to manmade tunnel or cave collapses – occur when acidic rainwater seeps down through surface soil and sediment, eventually reaching a soluble bedrock such as sandstone, chalk, salt or gypsum, or (most commonly) a carbonate rock such as limestone beneath. In a process that can last hundreds, sometimes thousands of years, the water gradually dissolves small parts of the rock, enlarging its natural fissures and joints and creating cavities beneath.

As the process continues, the loose, unconsolidated soil and sand above is gradually washed into these cracks and voids. Depending on how thick and strong that top layer is (sand will not last long; clay can hold out for millennia), and how close to the surface the void beneath is, the land may be able to sustain its own weight – and that of whatever we build on top of it. But as the holes grow, there will come a day when the surface layer will simply give way.

“Once those caves start to collapse, the materials above will simply funnel in,” says Dr Anthony Cooper, a principal geologist at the British Geological Survey, which maps the country for rock types susceptible to sinkholes and carries out surveys for developers, builders and individuals worried about the prospects of the land caving in beneath them. “It’s just like an eggtimer, really. That’s certainly what appears to have happened with this incident in Florida.”

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