CHEYENNE MOUNTAIN AIR FORCE STATION, Colo. - Cheyenne Mountain personnel watch as one of the mountain's 20-ton blast doors close during an exercise Oct. 25. The doors were shut for a 24-hour period during which personnel inside had to participate in a command-wide exercise. The 20-ton blast doors are closed and opened once a week. The last time they were closed for a contingency was 9/11, but they’ve never been closed for an exercise for more than 45 minutes. (U.S. Air Force photo by Joann Missey)