Since Sept. 11, 2001, the Defense Department has spent about $700 million to upgrade early-warning systems at the Cheyenne Mountain center. [Source]
“The Cheyenne Mountain center, at the eastern foot of the Rockies near the base of Pikes Peak, was constructed underground in the mid-1960s. Fearing nuclear attacks at the time, the United States built sites such as the Cheyenne Mountain complex. The Navy prepared a floating White House aboard the communications cruiser USS Northampton, in case the president needed to be evacuated from U.S. soil. Another protective bunker was created near White Sulphur Springs, W.Va., for members of Congress.”
I think military bases are really cool. Check out this underground Russian submarine base.