Briefing: The strange science of sinkholes

When a 6-metre-wide sinkhole suddenly opened and swallowed part of a house in Florida last week, killing a man, it seemed a freak accident. Four days later, a similarly sized hole opened just 3 kilometres away. What's going on? New Scientist takes a look at the forces at work behind this bizarre ? and sometimes deadly ? phenomenon.

What causes sinkholes?

They often occur in karst regions ? areas where slightly acidic groundwater dissolves the landscape's underlying bedrock, resulting in large subterranean voids. Most karst forms in carbonate bedrock like limestone or dolomite, though water can also eat away at gypsum and salt deposits to create such formations. Underground spaces can also form when water erodes buried sediment deposits, or where ancient lava tubes are covered over.

When the supporting material is removed, the sediment layers above become unstable and can suddenly collapse. Weather events can trigger them: if a prolonged drought dries out soil, a subsequent heavy rainstorm may be enough to send it crashing down. That is probably what happened in Florida last week.

Extended wet weather can be just as bad, as it adds more water to underground structures, weakening them. Severe tropical storms in 2012 may have helped trigger about 200 sinkholes that opened simultaneously across Florida, says Bricky Way of Geohazards, a company that detects underground voids and investigates sinkholes. Human activities like drilling or pumping out groundwater can also trigger sinkholes.

How common are they?

Regions prone to sinkhole formation are widespread in the US (see map). Harley Means, a geologist at the Florida Geological Survey (FGS), says karst regions are widespread around the globe as well. Such areas are fairly well mapped, but there is no authoritative database of where and when sinkholes have occurred. The FGS's data largely comes from property owners' reports on damage. "If one happens in the woods, that data doesn't make it to us," Means says, which makes it difficult to put an exact number on how many there are each year.

Sinkholes the size of last week's are not uncommon, says Way ? his company sees a few every year. But thankfully fatalities are rare. "In Florida, you've got a better chance of being bitten by an alligator," he says.

Are there more now than there used to be?

Sinkhole reports in Florida have tripled since 2006, suggesting that a "sinkhole alley" is ravaging the state. But that is probably just because they are being reported more often, says Means: Florida has one of the fastest growing populations in the US. Human activity in karst regions is increasing as well. Heavy construction activities and draining groundwater for drinking can make sinkholes more likely.

Could climate change result in more sinkholes?

It is possible, says Means, although there is no direct evidence of this. Torrential rainstorms and long droughts are both predicted to become more common in the next century as the planet warms, particularly in the south-eastern US. "It's speculation right now," says Means. He suggests that research that compares climactic conditions with periods of high sinkhole activity could provide evidence.

Can we predict sinkholes?

Not precisely, says Means, although there are ways to detect which regions are most likely to develop them. Some tools used by the Florida Department of Environmental Protection include ground-penetrating radar and electrical imaging, which allow geologists to look at the physical properties of the underlying earth strata to see if there is empty space there. Then they can drill into this area and test whether sediments are starting to fall in. "We can make a fairly educated guess that one may occur," says Means.

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