Early images of the U-2 spy aircraft being tested at Area 51

Area 51 was a covert CIA facility located in the remote desert of Nevada. For decades, the secretive place created a multitude of conspiracy theories about UFOs and aliens. But in reality the government workers stationed there were busy designing and developing high-altitude spy planes -- most notably the U-2 aircraft.

On Thursday, many of the details and history around Area 51 and U-2 planes were released in a recently declassified report by the CIA titled "The Central Intelligence Agency and Overhead Reconnaissance: The U-2 and OXCART Programs, 1954 - 1974." The National Security Archive got the CIA to declassify the report under the Freedom of Information Act.

Pictured in this photo is a U-2 plane in 1955.

According to the CIA report, the first U-2 was put in the air for a test flight in the summer of 1955. The U-2 spy plane -- known initially as just "the Article" -- quickly became a mainstay of U.S. reconnaissance and surveillance missions over the Soviet Union, Middle East, Cuba, and China.

Pictured here is a prototype of a U-2 at Area 51.

Famed engineer Kelly Johnson, most noted for his "Skunk Works" projects, designed the U-2 spy planes, which were manufactured by Lockheed. Here is a design of the aircraft with "trapeze" anti-radar attachments.

According to the report, U-2 designers came up against two central problems in developing the plane: fuel capacity and weight. To solve these dilemmas, the tail, wings, and landing gear were constructed unlike any other U.S. military aircraft.

The tail was attached to the main body with just three tension bolts to save weight; and the wings were ultra long, thin, and narrow. The landing gear was also lightweight with a set of wheels in the front and the rear of the aircraft. While on the ground, pogo sticks were used to prop up the U-2's wings.

Pictured here is the U-2 landing gear with the wing pogos.

When U-2 spy planes first soared over Nevada in 1955, they flew higher than any other plane ever: 60,000 feet. When people who lived nearby saw the sleek planes -- far above, glinting in the sun -- they had no idea what they were. And naturally, it became widely believed they were UFOs carrying aliens.

"If a U-2 was airborne in the vicinity of the airliner... its silver wings would catch and reflect the rays of the sun and appear to the airliner pilot, 40,000 feet below, to be fiery objects," the CIA report reads.

Pictured here is a U-2 detachment flying in formation over Nevada.

Area 51 was in a desolate corner of Nevada, where the CIA could conduct its flight tests in near secrecy. This map shows exactly where the facility was located.

This is a view of Area 51 that was pictured in the CIA report.

Here is a U-2 landing on the USS Kitty Hawk on August 5, 1963.

Once he developed the U-2, Johnson set to work creating the U-3. This image shows a sketch of the U-3 from Johnson's notebook. Before the release of the CIA report on Thursday, it was unknown what aircraft Johnson had drawn here.

The A-12, another reconnaissance aircraft designed by Johnson, was also tested at Area 51. This image shows A-12s parked on the ground at the remote facility.

Notoriety attached itself to the U-2 in May 1960 when the Soviet Union brought down a CIA flight over its territory. To cover it up, the U.S. issued a press release from NASA that said the U-2 was conducting weather research and strayed off course. To bolster the cover-up, a U-2 was quickly painted in NASA markings, with a fictitious NASA serial number, and put on display for the news media at the NASA Flight Research Center at Edwards Air Force Base. This story quickly blew up in the agency's face, however, when the aircraft wreckage was pictured by the Soviet Union proving it was a spy plane.

Pictured here is the fake NASA U-2 parked at Edwards Air Force Base.

In 2011, the U.S. Defense Department planned to retire the U-2 and have unmanned Global Hawk drones take over high-flying missions. But, that didn't happen. In fact, the planes are still very much in use today. This photo shows a modern-day U-2.