Just outside Durango, Colo., archeologist Rand Greubel stands on a mesa surrounded by juniper trees. He points to a circular hole in the ground, about 30 feet across and more than 8 feet deep. There's a fire pit in the center of an earthen floor, ventilation shafts tunneled into the side walls and bits of burned thatching that suggest how the structure once continued to rise above the ground. It's a large pit house from what's known as the Pueblo I period.
"We knew right away that it was highly significant just because of the sheer size of it," Greubel says.