Sinkholes are sudden, catastrophic, and almost entirely unpredictable and one Australian city sits on a rabbit warren of potential sinkholes.

In the last year alone they have caused houses, apartment blocks, roads and even a sleeping man, to disappear into the ground without warning – and every year there are more destructive sinkholes than ever before.

Experts say our cities are causing them and geophysicist Tim Bechtel is about as expert as they come — he grew up with a sinkhole in his backyard.

“Every year it would appear, this hole with about a ten-foot diameter. My father would fill it in and then every year it would appear again," Bechtel told Sunday Night.

Tim lives half an hour from “the sinkhole capital of the world”, the Pennsylvania town of Palmyra, where sinkholes happen on a weekly basis.

Florida also has one of the highest sinkhole rates in the world – the epicentre is a place they call “Sinkhole Alley” where more than 19,000 sinkhole cases have been documented. It was here in 2013 that cameras captured an entire apartment building collapsing as a sinkhole opened up beneath it.

Florida man Jeremy Bush told Sunday Night about the nightmare of having a sinkhole open under his house and swallow his brother, Jeff.

“We heard a loud bang – it was like a car or a train hitting the house. I ran into Jeff’s room and just saw a massive hole. I could hear him screaming for help and I just jumped in the hole and started digging with my hands trying to get to him.

"I didn’t even think about myself, I was just trying to get him out. You never imagine something like this could happen.”

Jeff's body was never recovered.

WHAT ABOUT AUSTRALIA?

Mount Gambier is one of a very small handful of places in the world with the soft limestone bedrock that creates deep, circular, spectacular sinkholes like the ones found in Mexico and Florida.

Beneath the regional centre of Mount Gambier is a myriad of caves. According to Aussie geologist and sinkhole buff, Ian Lewis, the only things keeping them from collapsing are the dry conditions and relatively stable water table.

“Mount Gambier only has a population of 20,000, but if you built a city there with a few million people, it would be the next Florida," Lewis said.

In addition to southern Australia’s Limestone Coast, the northern suburbs of Perth, parts of Canberra, Newcastle, the Nullarbor Plain and western NSW have have seen sinkholes appear in recent years.

Australia’s largest sinkhole has a lake at its floor and it is a popular tourist attraction, but our diving expedition to the bottom of two at Ewen's Ponds and Piccaninnie Ponds, which no one has ever been to the bottom of, revealed the caves that could be the next sinkholes waiting to happen.

THE ANATOMY OF A SINKHOLE

Bechtel explained to Sunday Night that if your home happens to be built over limestone, beneath it the ground will be “like Swiss cheese” and riddled with caves. Groundwater dissolves the limestone until, in the right conditions, all it takes is heavy rain or a faulty sewer to trigger a dramatic and dangerous sinkhole.

“A sinkhole is basically a hole underneath the ground that is slowly eating its way up. The roof gets higher and higher until the weight of someone walking over the top or a truck driving over collapses the roof.

"That vertical tunnel has probably been eating its way up for days, if not months or years.”

He also believes human activity is causing more sinkholes than ever before. In the right conditions something as simple as urban drainage, that concentrates water runoff in one place rather than dispersing it, can cause a sinkhole.

“Sinkholes happen naturally but very, very rarely. Without humans around, one sinkhole may occur in a lifetime," Bechtel said.

"It’s scientifically fascinating, but from a human standpoint it is very much a tragedy. I think people have a right to be very nervous. There are going to be a lot more catastrophic sinkholes.”



Special thanks to the SA Department of Environment, Water and Natural Resources for their assistance in the filming of this story.