The only way in or out of Natural Trap Cave is a 15-foot-wide hole in the ground that's almost impossible to see until you're next to it. Over tens of thousands of years, several thousand insufficiently wary animals plummeted more than 80 feet into the chilly, dim, cathedral-like cavern.
Their bones lie entombed, layer upon layer, in sediment as much as 30 feet deep.
Over the past two weeks, bucket after bucket, by rope and pulley, some 10-20 paleontologists, their assistants and a few spelunking experts have been hauling bones and bone-bearing sediment up into the sunlight. They used screens to sift out tiny remains from the dirt.
The best stuff has been packed in Rubbermaid containers for shipment to universities in the U.S. and even overseas, to the Australian Centre for Ancient DNA at the University of Adelaide, Breithaupt said.
On Friday, paleontologists from Des Moines University and elsewhere who've been camping out in the scrublands near the western flank of the Bighorn Mountains since July 27 plan to head home.
But the really cool part — positively identifying and dating what they've found — is yet to come, back in the laboratories where they can do more detailed work.
"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," said Breithaupt, a paleontologist for the U.S. Bureau of Land Management.
Scientists hope the cave's high humidity and temperatures that never top the 40s might even preserve genetic material of extinct animals from the days when massive ice sheets last frosted over much of the North American landscape.
The oldest remains could date back 100,000 years, Breithaupt said.
This was Breithaupt's second visit to the cave. Last time, during the previous excavations between 1974 and 1984, a tall stack of scaffolding helped researchers get in and out.
This time, they've had to rappel in and employ single-rope-technique climbing to get out.
Breithaupt said he saw a few bones down there. He declined to speculate on which species they belonged to: Field identification in paleontology is iffy, he said.
Anyway, this was more of a reconnaissance trip than a comprehensive excavation.
"The plan was to get a sense of the lay of the land above ground and below ground so they could proceed on this project in a very expeditious manner in future years," Breithaupt said.
Soon, the BLM will once again shut and lock a metal grate over the cave entrance to keep out modern-day animals and any more curious folks — at least until scientists can return for more detailed work next summer.