An Ancient Subterranean World
Beneath the flat, treeless expanse of the Nullarbor Plain lies an immense and ancient subterranean world. The plain itself is the world's largest single exposure of limestone bedrock, covering about 200,000 square kilometers. This limestone, a relic of a shallow seabed from the Miocene epoch, has been slowly dissolved by rainwater over millions of years, creating a karst terrain of sinkholes and caverns. Many of the cave systems formed between three and five million years ago during the Pliocene epoch. Australia's subsequent drift into a more arid climate effectively halted many of the geological processes, preserving these ancient features in a state of arrested development.
The caves often descend vertically before opening into vast horizontal passages and chambers. Many of these passages intersect the water table, creating enormous, still lakes of brackish water up to 90 meters below the desert surface. The water in these lakes can be stratified, with a distinct boundary layer, called a halocline, separating less salty water on top from denser, saltier water below. One of the most famous of these underwater systems, Cocklebiddy Cave, has a submerged passage more than six kilometers long, where divers have set world records for underwater cave penetration.