UNITED STATES - A declassified Central Intelligence Agency (CIA) document confirms the existence of Area 51.
But conspiracy theorists will be disappointed that it offers no proof of alien landings in the desert.
Area 51 has long been fodder for science- fiction films and wild UFO tales, which claim the US government imposed secrecy over the site north-west of Las Vegas to cover up evidence of extra-terrestrials.
Instead of encounters with UFOs, the documents released by the CIA on Thursday recount a less sensational history of Area 51 - as a testing range for the government's U-2 spy plane during the Cold War.
The report, entitled "Central Intelligence Agency and Overhead reconnaissance: The U-2 and Oxcart Programs, 1954-1974", makes no mention of the legendary "Roswell incident", when a weather balloon crashed in New Mexico in 1947.
UFO believers allege it was an alien spacecraft that went down, and that Area 51's hangars had hidden evidence of extraterrestrial corpses.
But according to the CIA, the government secrecy surrounding Area 51 was not about Martians, but about hiding a new spy plane from the Soviets.
The U-2 reconnaissance aircraft was designed to snoop on the Soviet Union at altitudes above 18,000m, and its development was top-secret.
In April 1955, the CIA chose a remote dry lake bed in the Nevada desert as a testing ground, which was designated on maps as Area 51.
Test flights for the U-2 aircraft were conducted at a much higher altitude than airliners or other military planes.
"High-altitude testing of the U-2 soon led to an unexpected side effect - a tremendous increase in reports of unidentified flying objects," the report said.
The reports of UFOs often came from pilots of commercial airliners in the early evening hours, with the U-2 plane's silver wings reflecting the rays of the sun.
The surveillance planes appeared to be "fiery objects" high in the sky, it added.
"At this time, no one believed manned flight was possible above 18,000m, so no one expected to see an object so high in the sky," it said.
Anxious to avoid exposing the ultrasecret U-2 programme, US Air Force officers explained the sightings as merely due to natural phenomena, though they knew the high-flying U-2 was the true cause.
U-2 and other surveillance flights "accounted for more than one-half of all UFO reports during the late 1950s and most of the 1960s," the report said.