A cave in Wyoming considered North America's premiere treasure trove of fossils from the late Pleistocene era is being reopened to paleontologists for research for the first time in 30 years, federal officials say.

Natural Trap Cave, 85 feet deep, is full of fossils of mammoths, bears, American lions and cheetahs that failed to notice the cavern's obscured 15-foot opening and fell to their death.

The bones of thousands of animals cover the floor of the cave in the north-central part of the state in a layer almost 30 feet deep.

The U.S. Bureau of Land Management says it is getting ready to reopen the metal grate covering the cave's opening to allow an international team of researchers access to the cave and its bones, some of which may be 100,000 years old.

The grate was installed in the 1970s to keep casual cavers and animals from falling into the cavern.

The only method of entering the cave is by rappelling down a rope, and the way only to get back up is an eight-story climb using the same rope, researchers say.

"One can only hope that, as a researcher, you're able to leave," says Bureau of Land Management regional paleontologist Brent Breithaupt. "It's an imposing hole in the ground. But one that actually has very important scientific value."

The interior of the cave is always cold and clammy, but those conditions are perfect for preserving remains, and there is even hope of retrieving fragments of DNA from the bones, says research team leader and Des Moines University paleontologist Julie Meachen, who was a college student when she last visited the cave the last time it was open to scientists.

"It's so cold all year long, that it has got just the perfect conditions for preserving DNA, in multiple species, in large numbers of individuals," she says. "Which is not really found anywhere except Siberia and the Arctic."

DNA analysis techniques were not available the last time the cave was open for research, and any new findings could help scientists understand how ancient animals inhabiting the region in the foothills of the Bighorn Mountains during the planet's last glacial period around 25,000 years ago were related to their modern counterparts.

The researchers say they will have a two-week window in which to explore the cave, unearth and bring to the surface as many fossils as possible.

From a nearby camp they'll enter the cave daily, using ropes to haul boxes of bones to the surface.

"I don't think it's necessarily going to be easy," Meachen says. "But I think we're going to be pretty well prepared."