Some of the world’s most important buildings have only been seen by a relatively small number of people. In other words, they are top secret buildings. Many of these locations house the headquarters of intelligence agencies, holding miles of classified documents and government secrets within their walls, or remain accessible only to scientists and researchers to protect the valuable contents. Agencies have taken drastically different approaches to the constriction of their secretive sites–some are hidden away from the public eye, while others stand out in the skyline of the city. From the doughnut-shaped headquarters of Britain’s GCHQ to a military command center tucked away in a Colorado mountain, the world’s most top secret buildings have histories as fascinating as what goes on inside of them.