Once you classify something as top secret, people find a way to fill in the void of information with speculation, which can turn into conspiracy theory, and might become legend. The U.S. military's Area 51 in Nevada is legendary for its secret research on captured UFOs and the aliens they contain. The legend grew because the government wouldn't explain the classified aviation research going on there. The acreage was set aside for the development of the U2 spy plane in the 1950s.
U-2 testing began in July 1955, and immediately reports came flooding in about unidentified flying object sightings. If you read the details in a 1992 CIA report that was declassified with redactions in 1998 (and subsequently released nearly in full in 2013), it's easy to see why.
Many of these sightings were observed by commercial airline pilots who had never seen an aircraft fly at such high altitudes as the U-2. Whereas today's airliners can soar as high as 45,000 feet, in the mid-1950s airlines flew at altitudes between 10,000 and 20,000 feet. Known military aircraft could get to 40,000 feet, and some believed manned flight couldn't go any higher than that. The U-2, flying at altitudes in excess of 60,000 feet, would've looked completely alien.
Naturally, Air Force officials knew the majority of these unexplained sightings were U-2 tests, but they were not allowed to reveal these details to the public. So, "natural phenomena" or "high-altitude weather research" became go-to explanations for UFO sightings, including in 1960 when Gary Powers' U-2 was shot down over Russia.
These obviously lame excuses fed speculation about extraterrestrials. As the years went by, more astounding aviation breakthroughs were achieved at Area 51, but the geniuses behind them got no credit because of the secrecy. Meanwhile, the legend has become a moneymaker for businesses surrounding the forbidden zone. Read the history of Area 51 at Popular Mechanics. -via the Presurfer
(Image credit: X51)