The first paleontological excavation in 30 years of a unique cave in northern Wyoming has yielded a treasure trove of fossils of creatures that inhabited ancient North America, researchers report.

After rappelling down to the bottom of Natural Trap Cave, the only method of entering from its almost-hidden 15-foot-wide entrance, researchers found fossilized remains of North American cheetahs, lions and short-faced bears, and at the other end of the size spectrum the exquisitely preserved bones of tiny rodents.

Over tens of millennia thousands of animals, failing to see the obscured opening of the cave, fell 80 feet down into the dark cavern, creating layer upon layer of bones 30 feet deep in places.

The National Park Service, responsible for the Bighorn Canyon National Recreation Areas where the cavern is located, has described the cave entrance as "virtually impossible to see until it is directly underfoot."

A group of paleontologists has spent the last two weeks using ropes, pulleys and buckets to haul bones to the surface.

"They're very excited about the potential for what they've found," says Brent Breithaupt, one of a group of scientists given access to the cave.

For the last 30 years the cave's entrance has been covered by an iron grate placed there by the Bureau of Land Management and intended to keep animals, and curious humans, from falling into the cave.

Before that, from 1974 to 1984, excavations were carried out in the cave.

Bones recovered in the current effort will be distributed to universities and research institutions in the United States and abroad for study and identification, Breithaupt, a BLM paleontologist, says.

"The analysis, yet, is still very preliminary," he says, explaining the positive identification and dating of the bones will come from more detailed analysis in various laboratories.

Some of the researchers say it's even possible high humidity and cool temperatures in the cave may have preserved genetic material from extinct species roaming North America when massive sheets of ice last covered it.

For that reason the Australian Center for Ancient DNA and the University of Adelaide will also be provided with samples from the current dig, Breithaupt says.

Some of the oldest remains might be as ancient as 100,000 years old, he says.

"It's an incredible site. It definitely is one of the most significant sites that BLM manages and it will provide very, very important information," he says.

The iron grate over the entrance will be shut and locked again soon, the BLM says, until scientists come back for even more detailed investigations next summer.