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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.