The Nullarbor Plain has a reputation for being flat and featureless, but 3D satellite images reveal there is much more to this ancient landscape than meets the eye.
An international team of scientists has discovered the relic of a reef, preserved for millions of years.
The team, led by Matej Lipar of the Anton Melik Geographical Institute in Slovenia, discovered the ring-shaped reef while hunting for meteor craters in satellite data.
The reef has a central dome about 500 metres wide surrounded by an outer ring 1,300m across, they report in the journal Earth Surface Processes and Landforms.
It is the first time a reef structure has been found on the Nullarbor, said Milo Barham of Curtin University's School of Earth and Planetary Sciences.
While the reef looks like a little bullseye on the satellite data, it's not that obvious up close in the landscape.
"You wouldn't necessarily see it and think, 'Oh my God, it's a reef,'" Dr Barham said.
"It's only maybe 3 metres to 10 metres higher than the surrounding plains."
How did the Nullarbor form?
The Nullarbor Plain is an ancient sea floor that gradually emerged from the ocean.
Stretching across 200,000 square kilometres of South and Western Australia, it is the largest limestone formation in the world.
"A lot of people may just blast across the Nullarbor and don't stop for anything because they think there's nothing worth looking at," Dr Barham said.
Despite its nickname – Nullarboring – the area is starkly beautiful, as well as being a geological time capsule.
After Australia and Antarctica split about 40 million years ago, bands of limestone were laid down as sea levels ebbed and flowed at three times.
"You see the different layers of limestone in the sea cliffs," Dr Barham said.
Then, about 14 million years ago, the sea floor was uplifted and tilted by the movement of tectonic plates.
"We can tell it was tilted because if you follow the ancient coastline … there is a 100-metre height difference between the north-west and the eastern sides," he said.
Later on, the area became very dry, creating a geographical barrier leading to the evolution of different species of animals and plants, such as eucalypts, in the east and west.
But the fact the Nullarbor now gets so little rainfall and has no rivers cutting across its surface means ancient landscapes etched into the bedrock have been preserved.
"If you look at satellite images, you see some faint river channels [that] were active maybe 2 or 3 million years ago on the northern side," Dr Barham said.
There are also the imprints of ancient sand dunes.
"You see these linear ridges in the limestone, these little hills, very gentle, only a metre or two in height."
While seismic imaging offshore had previously found evidence of reefs that formed before the seabed was lifted, none had been found on the plains before, Dr Barham said.
How do we know it was a reef?
After detecting the bullseye in the satellite data, the team travelled out to the Nullarbor to find out what it was.
"By using satellite images in combination with sampling, you start to reconstruct these environments over millions of years to see how they've changed and have been preserved much longer than you think they should."
Upon inspection, the team quickly ruled out a meteor crater.
"For an area [the size of the Nullarbor], you would expect to see impact craters, but none have been identified so far," Dr Barham said.
The researchers found no evidence of glass granules in the rocks, which would point to them having been modified by something slamming into them at high speed.
They also found the rock circle was the wrong shape and composition to have been created by wind, like the relics of the ancient dune systems found elsewhere.
"The only place we saw similar kinds of [structures] was on the Great Barrier Reef today."
The team polished down the rocks to the width of a human hair to see what was inside.
"Elsewhere in the region, you see limestone that would have formed in a seagrass meadow, but in this structure, we found micro-organisms that can bind sediment together."
Micro-organisms that do this, such as stromatolites and corals, are essential for building reefs.
And there are likely to be more features like this elsewhere on the Nullarbor, according to an early sweep of more satellite imagery.
'Compelling evidence'
Geologist Tom Raimondo, of the University of South Australia, said the study provided a compelling story about the origin of the Nullarbor's landforms.
"There are all sorts of cryptic records buried in the rocks," said Professor Raimondo, who was not involved with the discovery.
"And by virtue of the kind of datasets that we've got access to now, you can start to pull these [records] apart."
He said the discovery was made possible due to the precise spatial data from the 3D satellite, which reveals changes in the topography, such as height.
"If you look at any of the images, you'd be forgiven for not noticing this kind of vague lump," Professor Raimondo said.
But, he said, these kinds of variations were important, especially when looking at an area of the scale of the Nullarbor, where it is very difficult to detect subtle changes in the landscape.
While many people think our continent has been quiet over the past 20 million years, Professor Raimondo said places like the Nullarbor revealed a different story.
"The [uplift] created one of the most iconic coastlines — the Great Australian Bight — and was incredibly important to Australia's geomorphology," he said.
"You only have to scratch the surface to see there's a bit more going on than meets the eye."
