The world's largest limestone slab
The Nullarbor Plain is the largest single exposure of limestone bedrock on Earth, covering an area of about 200,000 square kilometers. This flat and treeless expanse was once a shallow seabed. The limestone itself, primarily the Eocene-aged Wilson Bluff Limestone, can be up to 300 meters thick. It is composed of the calcareous skeletons of marine organisms like bryozoans and foraminifera from the Middle to Late Eocene and Early to Middle Miocene epochs.
Over millions of years, a geological process called karstification has shaped a hidden world beneath the plain's surface. Rainwater, made slightly acidic by absorbing carbon dioxide from the atmosphere, seeps into cracks in the limestone. This weak carbonic acid slowly dissolves the rock, creating a massive underground drainage system. On the surface, this process forms features like sinkholes, or dolines, and blowholes—narrow tubes that "breathe" air as atmospheric pressure changes. Below ground, it has carved out hundreds of caves, some with passages extending for many kilometers. The initial formation of these major cave conduits likely began during the warmer, wetter climate of the Oligocene epoch, around 25 to 35 million years ago.
A vault of extinct giants
The sinkholes of the Nullarbor are natural pitfall traps. For hundreds of thousands of years, unsuspecting animals have fallen through the openings, their remains preserved in the stable, arid environment of the caves below. These caves contain one of the world's most large collections of Pleistocene megafauna fossils.
In 2002, cavers discovered a nearly complete skeleton of Thylacoleo carnifex, the marsupial lion. This formidable predator, about the size of a leopard but built more like a bear, had immensely powerful jaws and large, bolt-cutter-like premolar teeth. Subsequent expeditions have uncovered the skeletons of at least 11 individuals of Thylacoleo, along with giant short-faced kangaroos (Procoptodon goliah), Thylacines, giant wombats, and at least eight new species previously unknown to science. The remarkable preservation means many skeletons are found fully articulated, providing an clear look at Australia's extinct giants.
The caves also have a unique ecosystem. More than 280 species of invertebrates have been recorded, including at least 30 species of troglobites—animals fully adapted to life in complete darkness. These include blind cave spiders of the genus Troglodiplura, which are found nowhere else on Earth, and the blind cockroach, Trogloblattella nullarborensis.