Project Rio Blanco was an underground nuclear test that took place on May 17, 1973 in Rio Blanco County, Colorado, approximately 36 miles (58 km) northwest of Rifle. Three 33-kiloton nuclear devices were detonated nearly simultaneously in a single emplacement well at depths of 1,779, 1,899 and 2,039 m below ground level. The tests were conducted in fine-grain, low-permeability sandstone lenses at the base of the Fort Union Formation and theupper portion of the Mesaverde Formation.