The Remains of the Day: Among the greatest fossil sites in the world, Riversleigh and Naracoorte, though 1,200 miles apart, are linked by the astonishing insight they provide on the evolution of the continent's earliest creatures

The word epic barely does justice to these two fossil sites. There's the scope: Riversleigh's heat-scorched, aboveground fossil beds span 40 square miles, and Naracoorte covers about 2 square miles of cool pitch-black caves. There's the distance between the locations: Riversleigh is in the parched outback of far northwest Queensland, while Naracoorte is tucked just south of the lush Limestone Coast of South Australia. And then there is what makes them special paleontologically: Riversleigh offers a gigantic window in time, back 24 million years to when the continent was ruled by creatures so weird that scientists have given them names like Bizzarodonta and the Demon Duck of Doom. Naracoorte's treasure chest is equally dazzling, shedding light on more recent times. The scientific record locked within this labyrinth is unique. Animals dating back 500,000 years—including extinct giants such as the marsupial lion and 440-pound kangaroos—are preserved in mass graves, having been trapped after falling through tunnels and roof collapses. The caves also contain clues to the evolution of plants.

Riversleigh is the more remote of the two. The Riversleigh trail begins either 215 or 300 miles from the site itself (depending on which route you take), inside the Riversleigh Fossil Centre, in the mining city of Mount Isa. The complex has a visitors center with dioramas, a resident paleontologist, and—perhaps most important—facts on the region and the roads. The last is crucial, since driving to the fossil beds via the shorter route, on largely unpaved roads, takes about five hours and is best negotiated in a four-wheel-drive vehicle. The longer, better-paved route takes more than six hours, yet both traverse the seemingly endless red-earth landscape. Cell phone service is minimal, traffic is sporadic, and some of the dirt tracks are used by tandem trucks that can be more than 180 feet long. Tours to Riversleigh's D-site, the only fossil bed open to the public, leave from Adel's Grove, a private campground 30 miles away. Going it alone is a mistake: Without a knowledgeable guide, it's easy to stumble past the secrets held in Riversleigh's rocky outcrops.

The entrance to the more visitor friendly Naracoorte Caves National Park is six miles south of the town of Naracoorte (the parking lot—presided over by an elephant-sized model of Diprotodon australis, the largest marsupial ever to walk the earth—is hard to miss). The park has 26 limestone caves comprising passages and chambers both tiny and cathedral-like and marked by spectacular displays of stalactites and stalagmites. Some of these caves are dead-ended, terrifyingly convoluted, and chillingly cold. Others have been so opened by roof collapses that gardens have sprung up in them. A number are still sealed after hundreds of thousands of years. Tours range from the easy and self-guided (through the appropriately named Wet Cave) to the more challenging (requiring helmets, overalls, and a desire to squeeze through small gaps). The best is the three-hour, reservations-only World Heritage behind-the-scenes tour ($47), led by a senior guide conversant in everything from the region's history to the cutting-edge scientific research taking place inside these pits. Beginning in the Victoria Fossil Cave, the tour winds through passages and chambers and past ever more breathtaking rock formations like the Chess Chamber before reaching the Fossil Chamber, still groaning with bones and sporting a RESEARCHERS ONLY only sign that blocks public access. The tour guide removes the sign, and from here it's a quick, cramped crawl into a key fossil site, then on to areas such as the stalactite-striped Sunrise Scene. Better still, the guide takes visitors to see a dig in process. Few places have the capacity to humble the observer. Naracoorte is one.