For the first time in three decades, paleontologists 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 the bottom of a sinkhole-type cave. Natural Trap Cave in Wyoming is 85 feet (25 meters) 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 (4 meters) opening until they were plunging to their deaths. Now, the U.S. Bureau...