Area 51 is a heavily guarded military base roughly 100 miles northwest of Las Vegas.
Some people think the military is keeping captured aliens and their futuristic space machines there.
But the CIA has finally peeled back some of the secrecy surrounding the base. It released newly declassified documents that for the first time mention Area 51 by name and spell out how the base has used for developing cutting-edge aircraft like the Stealth Bomber and U2 spy plane.
"I certainly don't think there are aliens involved," said Jeffrey Richelson, a senior fellow at the National Security Archive.
Richelson is the man who pushed for the release of the CIA information. He says the document does remove some of Area 51's mystique, but probably not enough to quell UFO fans.
"I don't think it will sway people who believe that there are extraterrestrial aircraft out there patrolling the skies," said Richelson.
Area 51's mystery is a source of both pride and industry.
In Las Vegas, the minor league baseball team is called the "51s." And films like "Independence Day" have made millions off Area 51.
The mystique, though, may not really have much basis in reality. For some the mystery lives on, because "alien" theories can be awfully hard to knock down.