Green Waterhole-Tank Cave complex in South Australia's southeast rich in fossils uniquely preserved under water

A reconstruction, by Peter Schouten, of the ancient megafauna that were attracted to the Green Waterhole site in South Australia's southeast liestone cave region. At right: Griffith University associate professor Julien Louys has been among the team investigating the Green Waterhole-Tank cave complex underwater fossils.

The Green Waterhole-Tank Cave complex, 25 kilometres west of Mount Gambier in South Australia’s southeast, was placed on the state heritage list in 2022 for its underwater deposits of fossils preserved in a complete way almost unique in Australia.

The Green Waterhole contained the only known extensive underwater vertebrate fossil deposits in Australia and had preserved many extinct species of megafauna, with several new species recovered and awaiting description.

The combined Green Waterhole-Tank system, at 20 metres, was shallow compared to the region’s other sinkholes but it is the longest underwater cave system by far in South Australia. Formed about 125,000 years ago, provided an important opportunity to investigate the evolution of South Australian mammals and birds yet to be fully studied. Green Waterhole had yielded two named fossil bird species and 14 other birds. Of the birds, three are extinct and represent Pleistocene megafauna.

The underwater cave complex also hosted considerable mammalian fossil fauna, comparable in diversity to the megafauna in the world heritage Naracoorte Caves. This included a species of extinct short-faced kangaroo and propleopus oscillans, a nationally rare carnivorous and scavenging kangaroo from the Pleistocene (about 2.6 million to 11,700 years ago) era. The presence of many small bird fossils in the cave might have been from the standing water that attracted and helped preserve the remains of flocks of birds in an otherwise relatively arid period.

Being submerged allowed fossils to be preserved in an exceptional physical state, with several extinct kangaroos represented by complete or near-complete skeletons.

The heritage protection of the underwater caves came from experts at the South Australian Museum, Griffith University, Adelaude University, Flinders University and the Cave Divers Association of Australia working with the South Australian government environment and water department. Museum palaeontologist Dr Liz Reed, Griffith University associate professor Julien Louys and their colleagues were also awarded an Australian Research Council linkage project grant to investigate the the cave’s underwater fossils.

Louys said the challenges associated with accessing underwater fossils were "amazing" and they used the expertise and guidance of the Cave Divers Association of Australia. The association bought land at the entrance to Tank Cave system in 2010 to manage access.  The only other known natural entrance to the system was via Green Waterhole, managed by the environment and water department.

In 1998, the South Australian government recognised othe cave divers’ association’s modern training and limited access the caves to only its members. The cave divers association’s education programmes stressed conserving and preserving the site and the importance of minimal-impact diving techniques