Discovering backyard sinkholes | Photos

Mole Creek Farmer, Glen Anderson with water filled sink holes in the background. Picture: Geoff Robson
Mole Creek Farmer, Glen Anderson with water filled sink holes in the background. Picture: Geoff Robson

THEY are regularly on television and talked about on the radio, but did you know Tasmania has its own sinkholes?

More than 400,000 hectares of land across the state has either collapsed into sinkholes or has the potential for one to develop.

However, while the main kind presented in the media are catastrophic and sudden, that's not normally how a sinkhole occurs.

Department of Primary Industry, Parks, Water and Environment karst officer Rolan Eberhard said sinkholes developed in karst areas.

"What makes a karst different from other landscape is that there is interaction between what happens above the surface and what happens underground," Mr Eberhard said.

"Basically you've got movement of materials down in underlying areas, either caves or rivers, that are transporting material away, which can cause collapses."

Mr Eberhard said most sinkholes developed slowly and that sudden collapses were very unusual.

"Most often, a sinkhole will develop over hundreds of thousands of years as small amounts of material get moved from the base," he said.

"In Tasmania there is about 400,000 hectares of land that is undermined by limestone and karst rock types."

Mole Creek is a known karst area, as are Smithton, Gunns Plains, Mt Cripps, Hastings Caves and Junne Cave.

"At least half of the areas of limestone on karst rock are in reserve in the south-west," Mr Eberhard said.

The legend goes that Mole Creek's King Solomons Cave was discovered because a wallaby, being chased by a dog, fell into a sinkhole.

Once the sinkhole was discovered, so was the cave underneath it.

Marakoopa Cave at Mole Creek also has sinkholes above it, one of which is 50 metres in diameter and 100 metres deep.

Senior cave guide Haydn Stedman said the large sinkhole was discovered in 1906 and a visitors' walking track to the top of it was on the Mole Creek Cave's wishlist. It is a good 30 minutes' hike to reach the top of the sinkhole.

As it has been around for hundreds of years, it is full of vegetation, but still looks impressive.

A waterfall flows down into it and continues as Short Creek into the cave.

Mr Stedman said cavers often abseiled into the large sinkhole and that while it would be good to open it up to more people, they had to be careful about protecting it.

He said there was a chance more sinkholes had caves underneath, but it wasn't always the case.

"(Sinkholes) would all be linked with the water tables and the water system, but whether there's something big enough for us to call it a cave, is a different thing," he said.

Mr Eberhard said it would be great for somewhere like Mole Creek to make its sinkholes more accessible.

"The more opportunities people have to get close to and engage with these karst areas, the more people can see how special they are," Mr Eberhard said.

He said sinkholes should be protected by not using them as a dumping grounds or filling them with soil.

"The problem with rubbish is because these features are connected to the underwater ground system, if you put fuel drums, or batteries, or asbestos in, which people often do, that ends up in the water," he said.

He said a typical sinkhole was difficult to describe, as they were all very different, but the risk of any happening like in Florida, was very low.

"I'm not aware of any instances where people have been injured or a house or other building have been damaged," he said. "I never cease to be surprised in the differences in scales and forms that they show."


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