In 1955, the Central Intelligence Agency, U.S. Air Force and defense contractor Lockheed Martin chose an ultra-remote site in the Mojave Desert of southern Nevada, about 80 miles northwest of Las Vegas, to begin testing and developing the newest, most advanced aircraft in the world at the time.

For decades, the Nevada Test and Training Range, better known as Area 51, didn’t appear on any public map, and the U.S. government didn’t even admit it existed. Thanks to the ironclad security surrounding the site, and the experimental nature of the “black aircraft” tested there, rumors of unidentified flying objects, captive aliens and other mysterious activities have swirled around Area 51 ever since the ‘50s.

But even if no alien-made UFOs ever took to the skies over the salt flat known as Groom Lake, we now know—thanks in large part to declassified CIA documents—that a number of highly sophisticated, highly unusual aircraft were developed and tested there. From the U-2 spy planes of the Cold War era to a purely experimental Star Trek-inspired craft from the 1990s, here are six of Area 51’s most intriguing aircraft.

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U-2 Dragon Lady

Lockheed U-2 plane, 1957. Apic/Getty Images

In the early 1950s, at the peak of the Cold War, the CIA began a covert effort to develop a reconnaissance plane that could reach an altitude of 70,000 feet, high enough (it was thought) to avoid detection by Soviet radar. The result, developed under the code name Project Aquatone, was the U-2, the single-engine aircraft with glider-like wings designed by Clarence “Kelly” Johnson, founder of Lockheed Martin’s Advanced Development Projects division (better known as Skunk Works). Lockheed built the plane at Skunk Works headquarters in Burbank, California, in just eight months, then sent it for testing at Area 51, which Johnson nicknamed “Paradise Ranch.”

Before the U-2 was ready to fly, Lockheed engineers had to find a fuel that would not evaporate in the high altitudes at which the aircraft was designed to fly. To meet this challenge, Shell Oil Company produced a special low-volatility kerosene fuel using the petroleum byproducts it normally employed in its “Flit” fly and bug spray. In addition, the technology behind the pressurized suits developed to keep U-2 pilots alive at such high altitudes would later play a key role in the manned space program.

The U-2 (accidentally) took its first test flight over Groom Lake on August 1, 1955 and less than a year later flew over the USSR for the first time, becoming “immediately the most important source of intelligence on the Soviet Union,” according to a now-declassified CIA report. There was a cost, however: In 1956, three CIA pilots were killed during U-2 test flights, including two at Area 51 and one at an Air Force base in Germany. In May 1960, the Soviets shot down a U-2 over the Russian city of Sverdlovsk, capturing its pilot Francis Gary Powers and forcing the United States to admit it was spying. While President Eisenhower halted all U-2 flights over the Soviet Union, plans were already in the works for a smaller, faster—and stealthier—aircraft.

READ MORE: U-2 Spy Incident

A-12 Oxcart & SR-71 Blackbird

A-12 Oxcarts in a row, circa 1963. CIA

Launched in 1957, Project Oxcart produced two of the fastest, highest-flying aircraft in U.S. history, the one-seat Archangel-12 and the two-seat SR-71 Blackbird. The A-12 had two jet engines, a long fuselage and a distinctive cobra-like appearance.

The first completed A-12 arrived at Area 51 in February 1962, after being disassembled in Burbank and transported to Nevada in a specially designed trailer that cost almost $100,000 (more than $830,000 today). To keep the A-12’s existence secret, the CIA briefed the head of the Federal Aviation Administration (FAA), who made sure air-traffic controllers were told to submit written reports of unusually fast, high-flying planes, rather than mention such sightings over the radio. Still, reports of UFO sightings around Area 51 would reach new heights in the mid-’60s, writes Annie Jacobsen in Area 51: An Uncensored History of America’s Top Secret Military Base, starting just after the A-12 made its official first flight over Area 51 in April 1962.

Declared fully operational in 1965, after attaining a sustained speed of Mach 3.2 (just over 2,200 m.p.h.) at 90,000 feet of altitude, the A-12 began flying missions over Vietnam and North Korea in 1967. The following year, it was retired in favor of its Air Force successor, the SR-71 Blackbird.

A U.S. Air Force SR-71A, also known as the "Blackbird", is put through it''s paces during a test flight over Beale Air Force Base in California. The aircraft is a strategic reconnaissance plane by Lockheed and is the world''s fastest and highest flying operational aircraft. USAF/Getty Images

Longer and heavier than the A-12, the SR-71 paired supersonic speed with a low radar profile, due to its sleek tapered design and black radar-absorbing paint. On July 28, 1976, pilots flew an SR-71 at a record speed of Mach 3.3, or 2,193 mph. At 400 feet per second, this was literally faster than a speeding rifle bullet. Retired in 1990, after more than three decades of service, the SR-71 remains the world’s fastest aircraft.

READ MORE: Interactive Map: UFO Sightings Taken Seriously by the U.S. Government

Soviet MiG-21

USSR supersonic fighter jet MiG-21, seen taking off in October 1968. Sovfoto/Universal Images Group/Getty Images

In addition to testing new aircraft technologies, Area 51 was also used to study foreign warplanes that the U.S. government obtained covertly during the Cold War. In the late 1960s, according to now-declassified CIA documents, the Air Force obtained “Fishbed-E,” a Soviet MiG-21 jet fighter that was loaned to the United States after an Iraqi pilot used it to defect to Israel. Under the program codenamed Have Doughnut, Area 51 personnel inspected and reverse-engineered the Mach-2 fighter in order to learn how it performed and compare it to select U.S. fighter planes.

Over 40 days in 1968, U.S. pilots flew the MiG in 102 test flights, logging 77 hours of total flying time. They found that while the Soviet plane was slower than American planes like the F-5 and F-105, it had a tighter turning radius than any of them; this finding led analysts to warn U.S. pilots to avoid “prolonged maneuvering engagements,” or dogfighting.

The top secret MiG program at Area 51 paid dividends in the skies over Vietnam, where U.S. Air Force pilots ended the war with a two-to-one overall kill-loss ratio, downing a total of 137 Soviet-made MiGs. It would also spark the creation of the now-famous Top Gun fighter-pilot school, established in 1969.

READ MORE: 5 Cold War Close Calls

F-117 Nighthawk

An F-117 Nighthawk flying over the Sierra Nevada Mountain Range near Edwards Air Force Base, 2002. USAF/Getty Images

In the 1970s, Area 51 saw the development of the nation’s first stealth bomber, the F-117 Nighthawk, designed by Lockheed’s Skunk Works and developed under the code name Have Blue. With a faceted, diamond-like surface designed to reflect and disrupt radar beams, the F-117 could almost be mistaken for the boomerang-shaped UFOs that had been a fixture in the public imagination as far back as the 1940s.

Though the futuristic, alien-looking aircraft first flew over Area 51 in June 1981, it wasn’t publicly unveiled until late 1988, spending seven years under wraps as one of the Pentagon’s highest-value black projects. After bombing high-value targets across Baghdad to open Operation Desert Storm in early 1991, the F-117 served U.S. forces in Afghanistan and again in Iraq before it was retired in 2008. Still, an unknown number are still flying.

READ MORE: Are UFOs a Threat to National Security? This Ex-US Official Thinks So

Boeing YF-118G Bird of Prey

Boeing and the Air Force unveil the ultra secret Bird of Prey prototype. John B. Carnett/Bonnier Corp/Getty Images

In the 1990s, Boeing developed its own top-secret aircraft, the Bird of Prey, in a project managed by the Air Force at Area 51. A research-and-development aircraft that was never intended for production, the hawk-like YF-118G was named for its resemblance to the battlecruiser used by the Klingons in the 1984 movie Star Trek III: The Search for Spock. Its purpose was to test different aircraft technologies and ways to make planes less visible to the eye and detectable by radar.

The Bird of Prey first flew from Area 51 in 1996; it made 38 flights before the program was completed in 1999. It was declassified several years later, and Boeing donated the aircraft to the National Museum of the U.S. Air Force, though it continued to keep many of the plane’s most mysterious aspects under wraps.

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