The prospect of being enveloped by a sinkhole may be terrifying, but the science of why these large depressions occur is relatively simple.
The large hole that has suddenly appeared at Inskip Point, near Fraser Island in Queensland, Australia, is one of the more dramatic examples of how sinkholes can appear with little warning.
Queensland’s department of national parks said the sinkhole, which appeared on Saturday night, is now “relatively stable.” The cratering of the beach swallowed up a car, caravan, tents and a camping trailer and resulted in 300 people being evacuated.
The sinkhole is estimated to be 200 metres long, 50 metres wide and nine metres deep at its deepest point. Further erosion is expected to occur for several weeks and may even eventuate in the entire peninsula disappearing – albeit over the course of hundreds or thousands of years.
However, the sinkhole’s depth is relatively shallow compared to some overseas examples, such as when a man in Florida was enveloped by a 15 metre-deep sinkhole while happily tucked up at bed at home in 2013. Or the huge sinkhole that sucked down eight rare cars in a Corvette museum, also in the US.
There are a couple of main types of sinkhole once you eliminate direct human influences such as burst underground pipes or misguided digging.
Natural sinkholes occur when water, from rain or underwater streams, seeps through soil and sediment to soluble rock such as limestone, salt or gypsum, and components of some sandstones. The water gradually dissolves the rock, to the point where it cannot support its own weight or anything placed on top of it, and it gives way.
The most common type of sinkhole, which appears to have caused the Queensland incident, is caused by the collapse of underground caves, said University of Sydney geologist Armstrong Osborne.
“As the rock underground is dissolved by water seeping in, a cavity is formed and the roof of it becomes thinner and thinner,” he said. “This can be caused by a change in the underground environment or a redirection of water – someone left a hose on overnight near the Wellington caves once and woke up to find a two metre-wide sinkhole.
“The ceiling of the cave will usually be strong, an inch of rock can usually hold everything up. But then it can collapse. If the rock is close to the surface it is less dangerous but if it’s buried by sand you won’t know what’s going on.”
These dramatic collapses of underground caves pull down everything above them, making them particularly dangerous.
Luckily, Australia doesn’t have many urban areas built on soluble rock – Perth being an exception.
“Australia is relatively short of limestone, apart from areas like the Nullarbor,” Osborne said. “This is why sinkholes like this are quite rare, although this area of Queensland does have a history of them.”
Less reassuringly, there is no exact way of finding out if you are about to tumble into the maw of a giant sinkhole without detailed scientific analysis. Underground cavities can be detected by underground hazard mapping used before the construction of buildings. In some countries, the caves themselves are accessed to see how stable they are.
Geotechnical engineers are assessing the Inskip sinkhole using ground-penetrating radar.
“Our ultimate concern is about the safety of the visitors,” said Daniel Clifton, a senior ranger at the Queensland department of national parks.
“Until we get more information we’re not really sure about the stability of the site so we’re just being a bit precautionary.”
Osborne said: “We don’t have a lot of people doing geotechnical mapping in Australia, it’s a difficult thing to do.”
Without such mapping, there are few clues that a sinkhole is about to appear underneath your feet. If you’re lucky you may hear a noise, or some minor divots may appear first before the sinkhole envelops you.
“The edges are unstable so people are killed when many tonnes of material falls on top of them,” Osborne said. “You have to hope there’s another way out from the bottom but if the ground disappears I would run away very quickly. Your chances of survival aren’t good.
“It’s terrifying because we expect the ground to be stable under our feet. When it isn’t, it can be very scary.”
• This article was amended on 7 October 2015 because an earlier version said that sandstone dissolves in water. This has been corrected to say that components of some sandstones are soluble.
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