33. Royal Air Force Menwith Hill, U.K.

Although no one knows the current purpose of the Royal Air Force's facility, it was once used for intelligence gathering and espionage during the Cold War. The facility is 550-acres, and it is only used by ECHELON spies and the NSA. 

The facility opened in 1954, and it has driven conspiracy theorist mad for years. They continue to develop ideas about what goes on behind the barbed-wire fences and why they have Epcot-style bubbles all over.