Cheyenne Mountain Complex
The Cheyenne Mountain Complex (CMOC) is a non-public military installation of the United States Department of Defense located inside Cheyenne Mountain, in El Paso County, Colorado. It is the training complex for crew qualification, and is an alternate command center for the North American Aerospace Defense Command (NORAD), and United States Northern Command (USNORTHCOM).
Source: Wikipedia — Cheyenne Mountain Complex (CC BY-SA 4.0)