In the depths of a network of underwater caves, Julien Louys has been on the trail of some rather unusual animals.
Despite the sunken setting, these creatures weren't forms of marine life — they were giant marsupials, and they became extinct tens of thousands of years ago.
"For a long time, it was thought that all of these animals basically dropped off a cliff, they all became extinct at the one time," Dr Louys said.
"We don't really know."
Professor Louys — a palaeontologist at Brisbane's Griffith University, and an experienced scuba diver — has spent the past few days chipping away at that mystery.
Accompanied by members of the Cave Divers Association of Australia, and with a team of researchers waiting at the surface, Dr Louys has been searching caves near Mount Gambier in South Australia's south-east, where he has been collecting fossils.
The fossils belonged to ancient megafauna — a category that includes the two-and-a-half tonne marsupial diprotodon (incorrectly referred to as a giant wombat), the super-predator thylacoleo (the marsupial lion) and Palorchestes (sometimes called the marsupial rhino).
"The most common type of megafauna species that we find in this region are a group known as the sthenurines, which are short-faced kangaroos," he said.
"These creatures, some of them were up to maybe three metres in height and unlike modern kangaroos, we think some of them were actually walking rather than hopping."
Dr Louys said researchers were focusing on the underwater caves where fossils had been well-preserved until now, where modern diving equipment and techniques had rendered finding samples possible.
"In some instances the bones are just sitting on the surface of the floor of the cave and they've been sitting there since the animal died, and that's probably tens of thousands — if not hundreds of thousands — of years ago," he said.
Dr Louys said they hope to work out what kind of environment the animals lived in, by attempting to reconstruct their diet.
"One of the key things we're trying to address with this project is to really place these megafauna in a particular landscape, to reconstruct in as fine a detail as possible what the environments would have been like, and how that may have changed through time," he said.
They also want to work out the factors that led to their extinction.
"That's one of the most long-ranging debates in Australian palaeontology, and global palaeontology: what happened to these megafauna?
"They all became extinct towards the end of the Pleistocene and one of the key debates is whether it was humans that caused their extinctions or whether it was environmental change that caused their extinctions.
"There's been a lot of conjecture or a lot of hypotheses that humans may have caused the extinctions because climate changes weren't severe enough or weren't impactful enough.
"But one of the gaps in our knowledge of that time, and of these species, is what sort of environments they actually lived in and what sort of ecologies these megafauna species had."
