until shortly before the first occupation by native americans, the cavern was uninhabitable due to the large stream that occupied the entirety of the cavern. a rockfall from the cavern roof diverted the stream to one side, however, exposing a portion of the cave floor. the first occupants lived on this irregular floor of rock slabs. debris from occupants and deposits falling from the ceiling slowly raised the floor. the floor ultimately was raised an additional seven to eight feet (2.0 to 2.5 m) and up to 30 feet (9.0 m) near the upper entrance. in the 1960s, after the cave had been excavated, the united states bureau of mines installed 15 feet (4.5 m)-long bolts as supports to prevent additional collapses of limestone rock.