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Rob Morrison: This is Naracoorte, in the south-east of southern Australia. It's good farming country round here and it gets a lot of rain, but the rain grows more than crops. It's also limestone country, and limestone, hard as it is... ..can dissolve in rain over thousands or millions of years. It gets into cracks in the rock and widens them. Eventually, it forms big underground rivers, opening up huge cave systems underground. Perhaps the roof will fall in, creating a sinkhole. And then, having created these big holes, the rain - drop by drop - begins to fill them in again.

Professor Rod Wells works as a palaeontologist at Flinders University in South Australia and also for the South Australian Museum. And years ago, he discovered in one of these caves an enormous deposit of fossil bones of the extinct megafauna that once lived here. Since then, he's been putting the bones into skeletons and trying to work out how those ancient animals once lived.

Professor Rod Wells: The fascinating thing about this deposit, of course, was to work out how these animals came to accumulate in this cave, because there was no obvious entrance. But as we explored further through the cave, we found this great cone of sediment moving up towards the ceiling, and, in fact, this sediment had poured in through a hole in the roof of the cave. These animals had fallen down a pitfall just as the sediment was falling down the pitfall. And so the layers of sediment built up over time to form this great cone of sediment, encapsulating the remains of all of the unfortunate creatures that fell down the cave.

Indeed, these entrances to the cave, these holes in the ceilings, are like the shutter on a camera. They open and you get all the animals falling in, and if the sediment cone builds up fast enough and blocks that entrance, we have a snapshot of the fauna that lived at that period. If the sediment cone collapses and a hole opens again, more animals fall in, more sediment falls in, and the hole closes again. It's as if you shut the shutter on the camera again. And if this goes on through time, we end up with a series of what you might call 'multiple exposures' of that picture of the past fauna.

Rob Morrison: And there are other fossil deposits too in other caves that form part of the extraordinary network of caves in this region. It's a long way underground to get to them, and even down there, the going's not easy. You have to squeeze through small openings and crawl most of the way, but two hours later, you emerge at the fossil dig where Dr Gavin Prideaux is excavating bones from a different period.

Dr Gavin Prideaux: First thing we do is to take the trowel and gently scrape it across the surface of the sediment until we hit a bone. Once we hit a bone, we gently sweep the sediment away from around the bone with a brush and basically keep going with the trowel and the brush, working our way around the edge of the bone until we have it exposed. And the sediment that we pull out gets bagged up with a label telling us exactly whereabouts in the excavation it came from, and then we remove that from the cave and sieve it for all the small vertebrate remains.

The most important aspect to the study of this deposit is all of the small mammal and other vertebrate remains that we've actually sieved out of the sediments. Because those animals - rather than the larger animals - tend to be much more sensitive to environmental change, to climate change, through time, they actually tell us quite a lot more than the larger animals as to exactly what the climate of the time was like when the animals and the sediment were falling into the cave.

The thing about the Naracoorte Caves is that there are around about 25 caves in the area, and each one of them has a deposit of fossils of great importance, and the majority of those have not been studied. So, what we're trying to do here as part of the research program is build up a record for the last 500,000 years of vertebrate life in one area, and by studying all of those caves in detail, like we're doing here in Cathedral Cave, and putting all the information together, then we'll have that continuous record, roughly, for the last half a million years or so. And there's not another site in Australia nor in the world where one has such a detailed record.

Rob Morrison: Which is why these caves and the fossils they contain are now on the World Heritage register.

Professor Rod Wells: This continent has been an ark that's journeyed for 45 million years from its position near the South Pole to its position near the equator. And in that long journey, it evolved this unique flora and fauna unprecedented elsewhere in the world. And what we're getting a glimpse of here at Naracoorte is the last gasp of that great faunal radiation, and it's for that reason that this site has been made World Heritage.
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