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Paleontologists are wrapping up their first excavation in 30 years inside a bizarre cave in northern Wyoming thought to hold the remains of tens of thousands of ancient animals.

They're not entirely sure yet what they've found in two weeks of digging at Natural Trap Cave, but they're planning further study back in their labs.

Bones they've found could include those of North American lions, short-faced bears and other now-extinct species from 25,000 years ago.

The cave's only entrance is a 15-foot-wide hole in the ground that's almost impossible to see until you're next to it. Scientists say over the millennia, thousands of unwary animals plummeted 80 feet to their deaths within the chilly cavern.

A metal grate installed over the hole now prevents people and animals from falling inside.