The Secret Behind Area 51

Area 51, officially known as the Nevada Test and Training Range, is a classified U.S. Air Force facility located on the dry bed of Groom Lake in southern Nevada. The base has been the subject of UFO conspiracy theories since the 1950s, when test flights of the U-2 spy plane at extreme altitudes were being reported as unidentified aircraft by commercial pilots who had never seen anything fly so high. The CIA's existence of the base was not officially acknowledged until 2013. What is documented there includes the development of the U-2, the SR-71 Blackbird, and the F-117 stealth aircraft.

Source: Wikipedia