CHEYENNE, Wyo. (AP) -- For the first time in three decades, scientists are about to revisit one of North America's most remarkable troves of ancient fossils: the bones of tens of thousands of animals piled at least 30 feet deep at the bottom of a sinkhole-type cave.

Natural Trap Cave in north-central Wyoming is 85 feet deep and almost impossible to see until you're standing right next to it. Over tens of thousands of years, many, many animals -- including now-extinct mammoths, short-faced bears, American lions and American cheetahs -- shared the misfortune of not noticing the 15-foot-wide opening until they were plunging to their deaths.

Now, the U.S. Bureau of Land Management is preparing to reopen a metal grate over the opening to offer scientists what may be their best look yet at the variety of critters that roamed the foothills of the Bighorn Mountains during the planet's last glacial period around 25,000 years ago.

Des Moines University paleontologist Julie Meachen said she has been getting ready to lead the international team of a dozen researchers and assistants by hitting the climbing gym.

She hasn't done any real climbing before, she said, and the only way in is to rappel down. The only way out is an eight-story, single-rope climb all the way back up.

The cave is perpetually cold and clammy, with temperatures in the mid-40s and humidity around 98 percent.


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Even Bureau of Land Manage ment regional paleontologist Brent Breithaupt, who isn't one to get the willies from lots of animal bones, describes it as a tad creepy.

"One can only hope that, as a re searcher, you're able to leave," said Breit haupt, who visited the cave as a college student the last time it was open to scientists. "It's an imposing hole in the ground. But one that actually has very important scientific value."

Some mammal remains from the cave could be over 100,000 years old, Breithaupt said.

The remote site is exceptionally well preserved. It's far too challenging and dangerous to have been trammeled in by casual spelunkers. The Bureau of Land Management instal led the grate to keep people and animals out in the 1970s.