The legendary and until recently not officially existent ultra top secret military complex and dry lake runways of The United States Air Force facility commonly known as Area 51. A highly classified remote detachment of Edwards Air Force Base, within the Nevada Test and Firing Range. A base so secret, that even though its existence was known by the public, it was not officially acknowledged until 2005 by the CIA in response to a Freedom Of Information Act request.
How secret was this base? Well, in 1974 the crew of NASA's Skylab caused security chaos when they accidentally photographed the Groom Lake area from orbit, in what became known as the "Skylab Incident". Although the area remained nameless, a memo written in to CIA director by an unknown CIA official reported that there were specific instructions not to do this. <redacted> was the only location on the planet which had such an instruction. Astronauts on board Skylab 4 had inadvertently photographed a location that was considered by the CIA to be more sensitive than any other location on Earth.