A newly declassified CIA history from 20 years ago spills the story about Nevada's Area 51 and its secret mission — which was not to study UFOs, but to test the U-2 and other spy planes.

The CIA's story about the legendary test site is contained in "The Central Intelligence Agency and Overhead Reconnaissance: the U-2 and Oxcart Programs." The document was approved for release in June, with just a few remaining redactions, in response to a Freedom of Information request filed by George Washington University's National Security Archive back in 2005.

Much of the material was already known to Area 51 aficionados. "Nearly all of the newly released information is already in my books," British author Chris Pocock said in a commentary distributed by the National Security Archive. But the fact that Area 51 is explicitly mentioned in a publicly available document is nevertheless notable.



"It marks an end to official secrecy about the facts of Area 51," Jeffrey Richelson, a senior fellow with the National Security Archive, 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."

The book describes how officials involved in planning the spy-plane projects flew over the Nevada desert in a small plane in April 1955, looking for sites suitable for secret tests. "They spotted what appeared to be an airstrip by a salt flat known as Groom Lake, near the northeast corner of the Atomic Energy Commission's (AEC) Nevada Proving Ground," the book's authors wrote.

The facility had been used during World War II as an aerial gunnery range, and the officials decided it would be "an ideal site for testing the U-2 and training its pilots," according to the book. The AEC agreed to add the area to its real estate holdings, "and President Eisenhower also approved the addition of this strip of wasteland, known by its map designation as Area 51, to the Nevada Test Site."

A declassified map shows Area 51 in the Nevada desert. CIA via National Security Archive

The authors of the CIA history, Gregory Pedlow and Donald Welzenbach, said the site was nicknamed "Paradise Ranch," or simply the Ranch, to make it sound more attractive to the test project's workers. The first U-2 test flight took place at Area 51 on Aug. 4, 1955, and over the years that followed, the site was used for training U-2 pilots.

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The Groom Lake facility was also used for development of the U-2 spy plane's successors, including the Lockheed A-12 Oxcart and the D-21 Tagboard. Later on, Area 51 served as a test site for the F-117 stealth fighter. To this day, the area surrounding the facility has been closely guarded, and the airspace is off-limits to civilian air traffic.

Such high levels of secrecy, combined with the occasional sightings of strange aircraft, have fueled UFO tales for decades. For example, in a 2011 book titled "Area 51," investigative journalist Annie Jacobsen quoted her sources as saying that wreckage from the Roswell UFO incident ended up at Area 51 for study.

As one would expect, the CIA book makes no mention of Roswell or alien spacecraft.

Update for 8 p.m. ET Aug. 16: However, the book does mention that the U-2 flight tests created an "unexpected side effect — a tremendous increase in reports of unidentified flying objects." The authors said the spy planes flew at altitudes in excess of 60,000 feet, which was not thought possible at the time. Thus, air traffic controllers began receiving flurries of UFO reports from pilots who saw the planes above them.

The authors say lighting conditions in the early evening contributed to the UFO sightings: When the sun dropped below the horizon, as seen from 20,000 feet, the typical airliner would be in darkness. But the U-2 plane could still catch the glint of the sun at more than 60,000 feet.

The book said so many letters reporting UFO sightings were sent to the Wright Air Development Command in Ohio that the Air Force created Operation Blue Book to collect and investigate the reports. "Blue Book investigators regularly called on the Agency's Project Staff in Washington to check reported UFO sightings against U-2 flight logs," the authors say. "This enabled the investigators to eliminate the majority of the UFO reports, although they could not reveal to the letter writers the true cause of the UFO sightings."

The authors say U-2 and Oxcart flights accounted for more than half of the UFO reports during the late 1950s and most of the 1960s. However, UFO investigator Stanton Friedman disputed that claim in an interview with The Associated Press.

"The notion that the U-2 explains most sightings at that time is utter rot and baloney," he said. "Can the U-2 sit still in the sky? Make right-angle turns in the middle of the sky? Take off from nothing? The U-2 can't do any of those things."

Today, the U-2 is a museum piece, but Area 51 went on to serve as a testing ground for other secret aircraft. "Area 51 is where the drones came from — as we were saying way back when," Phil Patton, author of "Dreamland: Travels Inside the Secret World of Roswell and Area 51," told NBC News in an email.

Area 51 is also said to have played a role in the development of the modified MH-60 Black Hawk stealth helicopters that figured in the 2011 raid on al Qaeda leader Osama bin Laden's hideout. One of the scenes in the Oscar-winning movie "Zero Dark Thirty" plays out at a fictional version of Area 51.



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Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding +Alan Boyle to your Google+ circles. To keep up with NBCNews.com's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.