The star of the collection is Thylacoleo carnifex, the marsupial lion - a powerful, pouched predator with bolt-cutter teeth and a thumb claw, unrelated to any lion alive. Alongside it, the caves have yielded Diprotodon, the largest marsupial that ever lived, a wombat relative the size of a rhinoceros; kangaroos that stood taller than a person; and the thylacine, the striped marsupial wolf that survived into the twentieth century elsewhere before vanishing entirely. Victoria Fossil Cave alone has produced the remains of more than five thousand individual animals across nearly a hundred species. The fossil-bearing silt in places lies twenty metres deep, and dating it traces the assemblage from roughly 530,000 years ago up to the present.

Australia drifted away from Antarctica around 35 million years ago and has been an island ever since. Its animals evolved in isolation into forms found nowhere else, and Naracoorte captures that experiment in deep time better than almost anywhere. The caves also straddle one of the continent's great mysteries: the extinction of the megafauna. Within this record sits the moment, around 46,000 years ago, when the giants disappear - close to the window in which people first arrived on the continent. Was it climate, hunting, fire, or some combination? The bones at Naracoorte are central evidence in that unresolved debate, which is part of why, in 1994, UNESCO inscribed the park as a World Heritage site - one half of the Australian Fossil Mammal Sites, shared with Riversleigh in Queensland. It remains the only World Heritage site in South Australia.

The fossils would be reason enough to come, but the caves are also simply beautiful. The limestone formed underwater from coral and marine creatures, then was lifted and slowly dissolved by groundwater into chambers hung with stalactites and flowstone. Of the 28 known caves in the park, only four are open to visitors; the rest are reserved for research. The most famous chambers were transformed by a single discovery in 1969, when explorers broke into the vast fossil deposit in Victoria Cave and turned a regional tourist curiosity into a site of global scientific weight. Specially trained guides still lead adventure-caving trips into passages normally sealed to the public, helmet lamps swallowed by the dark.

Naracoorte is not only a tomb for the dead. Deep in the caves lives a colony of the southern bent-wing bat, a small, critically endangered species that breeds here in numbers and emerges at dusk to hunt. A purpose-built tour lets visitors watch the colony by infrared without disturbing it, and the bats' fortunes are followed closely, because their populations have been falling for reasons not yet fully understood. Above ground the park is unremarkable - flat, dry scrub where kangaroos, wallabies and emus graze. The wonder is all below, in the cool, still air of caves that have been quietly collecting the dead and sheltering the living for longer than our species has existed.