Officially it has never existed. Until now.

The CIA has for the first time acknowledged the existence of Area 51 - the enigmatic US airbase that is shrouded in mystery and is a staple of alien conspiracy theories.

The mysterious piece of land in Nevada has been linked with reports of UFOs, alien bodies and secret government projects.

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A declassified CIA history of the U-2 spy plane programme, made public this week, makes numerous references to the air base.

The documents came to light during research by the National Security Archive (NSA) at George Washington University for a report into the development, history and operation of U-2 spy planes during the Cold War.

As part of their research the archive obtained documents that for the first time actually make reference to Area 51. Included among the documentation, obtained through a Freedom of Information Act request made in 2005, is also a map showing the position of the secret base.

"President Eisenhower also approved the addition of this strip of wasteland, known by its map designation as Area 51, to the Nevada Test Site," the history reads.

To make the new facility in the middle of nowhere sound more attractive to his workers, Kelly Johnson called it the Paradise Ranch, which was soon shortened to the Ranch."

Much of the information contained in the newly released documents was already known to Area 51 aficionados, but the fact that the air base is mentioned in a publicly available document is nevertheless considered notable.

A public records request initially yielded documents, reviewed by National Security Archive senior fellow Jeffrey Richelson in 2002, but all mentions of Area 51 had been redacted.

Mr Richelson says he requested the history again in 2005 and received a version a few weeks ago with mentions of Area 51 restored.

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Officials have already acknowledged in passing the existence of the facility in central Nevada where the government is believed to test intelligence tools and weapons.

Mr Richelson believes the new document shows the CIA is becoming less secretive about Area 51's existence, if not about what goes on there.

"It marks an end of official secrecy about the facts of Area 51," Mr Richelson told The Las Vegas Sun. "It opens up the possibility that future accounts of this and other aerial projects will be less redacted, more fully explained in terms of their presence in Area 51."