Area 51, name for a U.S. military site, S Nev., c.80 mi (130 km) NNW of Las Vegas. Shrouded in secrecy since the mid-20th cent., it is located within the U.S. Air Force's enormous Nevada Test and Training Range and has long been used for research and development of secret experimental aircraft and weapons systems. A portion of Area 51 has been used for testing of nuclear weaponry since the Manhattan Project, and the base was of particular strategic importance during the cold war. The secretive nature of the installation and its flights of odd-looking experimental aircraft have undoubtedly contributed to Area 51's association with UFO sightings and a variety of theories involving aliens and other extraterrestrial phenomena.
See A. Jacobsen, Area 51: An Uncensored History of America's Top Secret Military Base (2011).