The Reef That Became a Mountain: The Nullarbor's Subterranean World

The Nullarbor Plain covers 200,000 square kilometres of southern Australia without a single permanent stream, a white limestone sheet so flat that the transcontinental railway runs straight for 478 kilometres. But beneath that blank surface lies one of the most extensive cave systems on Earth, where the bones of long-extinct megafauna have rested for 500,000 years.

The Sea That Built a Desert

Fifteen million years ago, during the Miocene epoch, a shallow tropical sea covered what is now the Nullarbor. The water teemed with foraminifera, bryozoans, molluscs, and corals, whose calcium carbonate skeletons rained onto the seafloor in an unbroken accumulation. Over millions of years, those skeletons compacted into limestone — a soft, porous rock that would later dissolve like sugar in rainwater.

The sea withdrew around 5 million years ago as the Australian plate drifted north into drier latitudes. What remained was a flat limestone platform, tilted slightly toward the coast, with no rivers to carve it. Rain did not run off; it sank.

That single fact — the absence of surface drainage — determined everything that followed.

The Dissolving of a Continent

Rainwater, slightly acidic from atmospheric carbon dioxide, percolated through the limestone, dissolving it along joints and bedding planes. Over millions of years, the water carved a labyrinth of tubes, chambers, and shafts hundreds of metres below the plain. The Nullarbor now contains more than 1,500 known caves, with new ones discovered every year.

The caves are unusual. Many are horizontal passages that follow the original bedding of the limestone, created where the water table once stood. As the plain slowly rose and the climate dried, the water table dropped, leaving the caves empty and dry. Without dripping water, few stalactites or stalagmites formed. The caves remained pristine, their floors covered in fine dust and the bones of animals that fell in from above.

The Nullarbor caves are not decorated like the show caves of other continents. They are cathedrals of absence — empty, silent, and perfectly preserved.

One of the deepest, Cocklebiddy Cave, descends more than 100 metres below the plain and extends for 6.5 kilometres. At its end lies a subterranean lake, its water unchanged since the last ice age.

The Fossils That Fell From the Sky

The caves acted as natural traps. For half a million years, animals wandering across the plain — often at night, unable to see the dark opening — stepped into the void. Their bones accumulated on the cave floors, sheltered from the elements and undisturbed by scavengers.

In Thylacoleo Cave, named for the marsupial lion it contained, researchers have recovered the most complete assemblage of Ice Age megafauna ever found in Australia. The remains include Thylacoleo carnifex itself, a leopard-sized marsupial with bolt-cutter teeth; Procoptodon, a giant short-faced kangaroo that stood two metres tall; Diprotodon, the wombat the size of a rhinoceros; and Megalania, a monitor lizard longer than a car.

Radiocarbon and uranium-series dating show that these animals lived between 500,000 and 200,000 years ago — well before human arrival. None of the bones show signs of butchery or fire. The cave preserved a pure record of natural extinction, driven by climate change as Australia dried and its forests shrank to desert.

A Landscape in Reverse

On the surface, the Nullarbor is often described as featureless. But the plain is not without topography. It is pitted with thousands of shallow depressions called dolines, where the limestone roof of a cave has collapsed. From the air, these sinkholes pattern the plain like craters on the moon.

The most dramatic is Murrawijinie Cave, a collapse that opened a window 30 metres deep into the limestone. Aboriginal people used its overhangs for shelter; their hand stencils still mark the walls, painted with red ochre that researchers have traced to the Flinders Ranges, 700 kilometres east.

The Nullarbor teaches a strange geology: the most important features are invisible. The water that shaped this landscape never ran above ground. The bones that tell the story of Ice Age Australia lie sealed in chambers no river ever reached. And the plain itself, so monotonous to the eye, is a reef — a seafloor raised to the sky, still dissolving grain by grain. Every year, the rainfall eats another millimetre of limestone, widening the cracks, deepening the caves. The Nullarbor is not finished. It is still being made.