Newly declassified CIA documents refer openly to the top secret piece of land in central Nevada. The base is also known as the Groom Lake facility and is located on the Southern shore of the lake.

The acknowledgement comes in a US government history of the U2 spy plane programme, obtained through a public records request. It includes an uncensored map of the previously 'secret' site. Area 51 was chosen as a base to test the jet in the 1950s, but until now was never identified on any government map.

"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.

"The outlines of Area 51 are shown on current unclassified maps as a small rectangular area adjoining the northeast corner of the much larger Nevada Test Site. 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."

Officials have already acknowledged in passing the existence of the facility, but some believe the new document shows the CIA is becoming less secretive about the region - although there has as of yet been no official confirmation or denial of some of the site's more controversial alleged activities.

Area 51 has been one of the most infamous US government facilities for decades. The secretive nature of the base has meant it has been at the centre of many conspiracy theories: typically related to classified technology projects, but sometimes associated with UFOs and alien investigations.

In pop culture, Area 51 is very often linked with the alleged UFO crash in Roswell, and the base has made appearances in a range of sci-fi books, films and video games.