Officially it didn't exist, but for generations it has featured in countless novels, films, comic books and the minds of conspiracy theorists as the place where the US government keeps crashed alien spaceships It's also where alien autopsies are performed and where the mysterious "men in black" are based, a secret force to keep the earth safe from extraterrestrials. Government officials have previously only mentioned Area 51 in passing, but now a newly declassified history provides the first official acknowledgement of its existence and provides details of what it has been used for - and it is not what the conspiracy theorists might want to hear. Instead, it turns out that Area 51, a patch of ground near Groom Lake in the Mojave desert, was used for the rather more mundane purpose of testing ultra- secret military aircraft technology - or so Washington says. According to the seven-chapter history, the space was used as an aerial testing ground for US government projects. The released documents specifically refer to the U-2 and Oxcart aerial surveillance programmes. "High-altitude testing of the U-2 soon led to an unexpected side effect - a tremendous increase in reports of unidentified flying objects," or UFOs, according to the documents, which became public through a Freedom of Information Act request by George Washington University's national-security archive. The documents, published in 1992, attributed the sightings to the U-2's ability to fly at more than 21,300 metres, considered by the public as too high for manned flight at the time. "U-2 and later Oxcart flights accounted for more than one-half of all UFO reports during the late 1950s and most of the 1960s," according to the documents. The Lockheed U-2 was tested at Area 51 rather than the nearby flight test centre at Edwards Air Force Base because officials did not want to arouse suspicion. The plane has a 30-metre wingspan but climbs so steeply after take-off that it quickly becomes invisible to the naked eye from the ground. U-2 aircraft have been used to conduct surveillance since the 1950s and were used extensively to spy on the Soviet Union, China and Cuba during the cold war. With regular technology upgrades to their sensors, the aircraft are still in use, detecting nuclear missiles, or hunting for roadside bombs in Iraq and Afghanistan. The U-2's 21-kilometre cruising altitude was originally meant to allow the plane to evade anti-aircraft missiles, but it now works well to collect phone and radio transmissions that would otherwise be blocked by the Afghan mountains. Its cameras can also take detailed pictures of potential trouble spots.