Cheyenne Mountain Air Force Station (NORAD), Colorado

Underground command center for monitoring the skies and space for hostile incoming weapons. Cheyenne Mountain was built with the knowledge that it would be a primary target for a nuclear attack, and it is perhaps the most fortified large underground installation in the world. Even so, the military acknowledges that it could not withstand a direct hit from one of its own nuclear missiles. The installation consists of 15 steel buildings, laid out in a 4.5 acre grid inside the mountain, and accessed through a tunnel and 23-ton blast doors. The buildings are suspended on 1,300 47" steel springs to absorb the shock of a nuclear detonation. 30 days of supplies and six million gallons of water are stored inside the installation, which underwent a major $1.7 billion renovation, following its 30th birthday in 1995. About 350 people work inside the mountain. Cheyenne Mountain Air Force Station is owned and operated by Air Force Space Command, who is its largest tenant. Although the Command Center located within the bowels of Cheyenne Mountain was rechristened as the NORAD and USNORTHCOM Alternate Command Center in 2008, the bulk of their daily operations occur at nearby Peterson Air Force Base, where their headquarters are located. There is a similar facility at Offutt Air Force Base (headquarters of United States Strategic Command), in Nebraska that can be used in case of a mission failure at Cheyenne Mountain.