The Beach That Became a Fossil of a Drowned Land: Western Australia's Eucla Basin Nullarbor Limestone

The Nullarbor Plain is the largest single piece of limestone on Earth, a slab of white rock the size of Italy that was once the floor of a warm, shallow sea. Today it sits 200 kilometres inland, a fossil of a drowned continent, its surface pocked with sinkholes and its caves filled with the bones of Ice-Age marsupials that fell into them.

The Sea That Built a Desert

Twenty-five million years ago, as Australia drifted north from Antarctica, the southern margin of the continent was submerged beneath the Eucla Basin—a broad, shallow seaway stretching from what is now South Australia into Western Australia. For millions of years, the shells of tiny marine organisms called foraminifera rained onto the seafloor, accumulating in layers that would become the Nullarbor Limestone.

The limestone is remarkably pure, composed almost entirely of calcium carbonate. It reaches thicknesses of up to 300 metres in places, a slow accumulation that took place over roughly 15 million years. When sea levels fell and the continent rose, the seabed emerged as a flat, arid plain—so flat that the Trans-Australian Railway runs for 478 kilometres without a single curve.

The Caves That Caught the Fallen

Rainwater, slightly acidic from atmospheric carbon dioxide, began dissolving the limestone as soon as it emerged. Over millions of years, this created one of the world's most extensive cave systems—more than 250 known caves and blowholes, many still unexplored. The caves formed along joints and fractures in the limestone, their chambers widening as groundwater ate away at the rock.

These caves became natural traps. For tens of thousands of years, animals blundered into hidden openings in the plain, falling to their deaths on the cave floors below. The dry, stable conditions inside the caves preserved their bones remarkably well, creating a fossil record that spans the last 500,000 years of Australian megafauna.

The Nullarbor's caves are time capsules: each chamber contains a snapshot of the animals that lived on the plain above, sealed in place by the slow drip of limestone.

The Bones Beneath the Plain

The most famous of these fossil deposits lies in Thylacoleo Cave, named for the marsupial lion whose skeletons were found there. The cave contains the remains of at least 89 individual animals, including giant wombats the size of hippopotamuses, short-faced kangaroos that stood three metres tall, and the marsupial lion itself—a pouched predator with bolt-cutter teeth and opposable thumbs.

What makes the Nullarbor fossils extraordinary is their completeness. Animals that fell into the caves were often preserved whole, their bones never scattered by scavengers or weathered by the sun. In some chambers, researchers have found the skeletons of entire families of giant kangaroos, suggesting that whole groups perished together when the ground gave way beneath them.

The caves also preserve evidence of the climate shifts that drove these animals to extinction. Pollen grains trapped in cave sediments record the spread of arid-adapted plants across the plain, a drying trend that coincided with the arrival of humans and the disappearance of the megafauna around 45,000 years ago.

A Landscape Still Changing

The Nullarbor is not a dead landscape. The limestone continues to dissolve, new caves form, and old ones collapse into sinkholes. In 2019, a team of cavers discovered a new chamber beneath the plain, its floor littered with the bones of animals that had fallen in thousands of years ago, their teeth still sharp and their skulls intact.

The plain itself is slowly eroding, retreating at its southern edge where the Great Australian Bight gnaws at the limestone cliffs. The sea that built the Nullarbor is reclaiming it, one wave at a time.