A UNIQUE sinkhole in the middle of a state highway linking Mount Gambier and Port MacDonnell has been attracting cave divers to the South East for many decades.
Known as “The Middle of the Road Hole” the Allendale Sinkhole is located in the heart of the small Allendale East township where the Riddoch Highway – locally known as Bay Road – splits centrally and diverts around the hole’s eastern and western sides.
Scores of unsuspecting motorists drive past the sinkhole everyday without an idea of what lies beneath.
With a 10 metre diameter at the surface, the hole features a steep, rough set of steps down its northern side, leading to a crescent-shaped pool of crystal-clear water around six metres below ground level.
A small dry-crawl passage can be found under the south-eastern wall, but it is the underwater region that is of special interest to cave divers.
Although the water at the entrance lake is very clear, it looks quite dark from the outside as it is shaded from direct sunlight.
Once under the water’s surface, the diver enters a single, slightly narrowing passage leading down a rock pile to a minor restriction at around 15 metres.
From there the passage opens into a larger chamber which reaches a maximum depth of 28 metres and would be passing well under the road overhead.
If the cave has been left undisturbed, visibility is usually excellent throughout its total length of around 35 metres.
Although the site is a sinkhole, it is classified “cave” by the Cave Divers Association of Australia (CDAA) since the lower regions of the second chamber are completely void of natural light.
Venturing down the rock pile from the lake entrance, and passing through the restriction, a diver would notice a workstation slightly to the left which was used by early surveyors.
It is in the form of a large, rectangular limestone brick and is nowadays decorated with a few empty beer bottles, rocks and other items collected and arranged by visiting cave divers.
Further down, the rock pile reaches a black silt floor with the steel wheel of an old wheelbarrow sitting on a rock.
The cave roof is stepped (three steps) from the restriction to the cave’s deepest point where graffiti from earlier divers can be found etched on the rock wall.
Here a narrow passage leads further down, but as far as could be determined only for a few metres.
The passage is far too narrow to turn around in, which means the diver must wiggle out – feet first.
On the way back to the entrance, just past the restriction at around 12 metres another flat passage can be found to the left.
This one is accessible and an interesting little diversion on your way out.
It leads around the edge of the rock fill towards the southern passage that was sealed by the road fill.
According to veteran cave diver and CDAA life member Peter “Puddles” Horne very little aquatic life is to be found in the cave.
“The only life-forms recorded to date are some syncarids and yabbies – including at least one pure white one,” Mr Horne told The Border Watch.
He said the cave was mapped in detail by Victorian cave divers Stan Bugg and Brian Cornell of the South Australian Underwater Speleological Society in the early 1990s.
“It has been known since before the beginning of the 19th Century, when it was apparently used as a major watering hole for bullock wagons.
“The August 3, 1971 edition of The Border Watch reported the cave was supposed to have appeared one night after a bullock wagon left one of the hotels in Allendale on its way to Port MacDonnell.
“The wagon, with its team and teamster, disappeared as the hole appeared, but the whole outfit was supposed to have surfaced again at Eight Mile Creek.”
Mr Horne said the origin of the large rock piles are numerous filling attempts over the years.
“One of the first attempts involved tonnes of the biggest basalt rocks the then Highways Department could handle, before many truckloads of loose paddock-stones followed.
“They only admitted defeat when the cave opened up again after about 9,000 tonnes of rock had been dumped into its entrance.”
Mr Horne said unfortunately no records of any pre-fill scuba dives have been located to date.
“It seems unlikely anyone dived it when one considers that the first recorded dives in the other sinkholes of the region took place during the very late 1950s,” he said.
Diving the cave is restricted to cave divers with CDAA accreditation.
As a child growing up in Mount Gambier in the 1950s and 60s Cave Divers Association of Australia science officer Ian Lewis recalls a “prison-like wall” around the Allendale Sinkhole.
“The wall was thick and around two metres high with broken glass on the top,” Mr Lewis said.
“It was made from large limestone bricks, some of which can still be found in the rubble pile in the cave today.
“The bricks were simply dumped into the cave when the wall was demolished.”
Mr Lewis said the wall was built to keep people and cattle out, but the bricks were by no means the only material used in an attempt the fill and stabilise the sinkhole.
“Initially limestone was used to fill the hole, but it didn’t really work because groundwater slowly dissolves limestone.
“Authorities then switched to basalt which worked better, but the hole still fell in again in the 1970s.”
In terms of its hydrogeology Allendale Sinkhole forms part of the Mount Schank – Ewens Ponds Groundwater Trough which runs roughly from north to south between the two and through to the coast.
The trough is characterised by many fractures in the limestone base of the region.
“Groundwater moves along these fractures before it reaches the ocean through Ewens Ponds, which is the biggest outflow to the sea in the region,” Mr Lewis said.
“Basically groundwater collects in this region before it flows out to sea.
“That explains all the little springs near the coastline in this part of the world.”
Mr Lewis said the sinkhole is an integral part of the region’s history and the story about the bullock and wagon disappearing in it has spread as far as New Zealand.
“I started to tell the story at an international conference there, when audience members interrupted me.
“They said they already knew the wagon and bullock mysteriously reappeared at Eight Mile Creek.
Some had even heard it washed up in New Zealand,” he said.
The sinkhole also featured on the cover of the 1973 South East phone book as a watercolour painting of the the bullock and wagon disappearing into the sinkhole.
“It was painted by a local student as part of a school painting competition,” Mr Lewis said.
