A vast stretch of submerged ancient sand dunes, found off the east coast of Australia, have scientists scratching their heads.

Key points: Dunes similar in shape and extent to those on Fraser Island drowned 12,000 years ago surprisingly well preserved

Dunes similar in shape and extent to those on Fraser Island drowned 12,000 years ago surprisingly well preserved Most dunes today have sand made of quartz but the ancient dunes contained calcium carbonate grains that cemented together after being exposed to rain

Most dunes today have sand made of quartz but the ancient dunes contained calcium carbonate grains that cemented together after being exposed to rain This helped the dunes remain intact when they disappeared under the water

The dunes are remarkably preserved, despite being drowned by rising seas.

Scientists think this is because they turned to stone before disappearing under the waves 12,000 years ago.

And they are like an ancient version of the World Heritage-listed Fraser Island sand dunes nearby.

"The site would have been an island exactly like Fraser Island, surrounded by water but 40 kilometres away from the current shoreline," said Tiago Passos, a PhD student at the University of Sydney who was part of a team that made the discovery.

The 70km stretch of dunes, discovered on the sea floor between Fraser and Moreton islands, has stumped scientists, who say such ancient structures should have been destroyed by rising water.

"I think the intriguing thing is what mechanism enabled them to be so well preserved," said Mr Passos, whose study on the dunes has been accepted for publication in the Australian Journal of Earth Sciences.

These images collected using sonar revealed the shape and extent of the 70-kilometre stretch of dunes. ( Supplied: Tiago Passos/University of Sydney )

Humans would have been around at this cooler and drier time, and could well have felt the sand of these dunes between their toes.

But after the dunes hardened, walking over them would have felt like walking on a rock, Mr Passos said.

The dunes were first discovered by an expedition to the Great Barrier Reef that mapped the sea floor at a location called Barwon Bank on the Fraser Shelf, 50km north of Moreton Island.

Sonar imaging picked up curved underwater structures 60 metres below the waves.

"They were originally thought to be part of an ancient reef," said Mr Passos, who analysed the images and samples of what turned out to be rock-like structures.

Researchers saw similarities in the ancient dunes with those from Fraser Island shown in this diagram. ( Supplied: Tiago Passos/University of Sydney )

Mr Passos's investigation revealed these curved features were not a reef but in fact dunes made of coarse grains of sand.

He said the underwater dunes are almost identical in shape to those on the nearby Fraser Island.

"This makes our case stronger that these features are sand dunes," he said.

But, he added, there is one major difference between the famous Fraser Island dunes and their underwater counterparts.

The 17,000-year-old sand grains in the submerged dunes contain calcium carbonate rather than quartz, which makes up most of the world's sand dunes today.

Mr Passos and colleagues argue the dunes survived the ages because, over time, the carbonate sand grains were cemented into rock through physical and chemical changes — a process called "diagenesis".

"Contact with fresh water in rain solidified the sediments," Mr Passos said.

Samples of the hardened dunes were also analysed and were found to be made up of grains of sand cemented together. ( Supplied: Michael Kinsela/University of Sydney )

An ancient version of Fraser Island

Mr Passos said the "palaeo-dunes" would have formed part of an island "analogous" to Fraser Island, which, at 122km long, is the biggest sand island in the world today.

Unfortunately, the parabolic Fraser Island dunes or any other present-day dunes are not likely to also survive being inundated by rising seas, he added.

"They would be eroded as they don't have enough carbonate content to turn them into rock," he said.

3D view showing the Fraser Shelf centred on Barwon Bank, which is 50 kilometres north of Moreton Island. ( Supplied: Robin Beaman/James Cook University )

Marine geologist Dr James Daniell of James Cook University, who was not involved in the research but has collaborated with some of the authors, said the dunes were "pretty significant features".

"Some of them are 10 to 15m high," he said.

"What's special is they are very similar to what we see on modern-day Fraser Island and Stradbroke Island."

Dr Kevin Welsh from the University of Queensland, who studies the formation of quartz sand dunes at places such as Cooloola and Fraser Island, agreed.

He said the discovery could help scientists understand how such sand dunes form.

"It shows these parabolic sand dunes can form under quite different conditions and with different materials," he said.