An unidentified flying object or UFO is defined as any perceived object in the sky not identifiable by standard criteria. However, the term is widely used for claimed observations of extraterrestrial spacecraft.
Early reported sightings of unknown flying saucers usually described them as silver or metallic, sometimes reported as covered with navigation lights or surrounded with a glowing light, hovering or moving rapidly, either alone or in tight formations with other similar craft, and exhibiting high maneuverability. While disc-shaped flying objects have been interpreted as being sporadically recorded since the Middle Ages, the first recorded use of the term “flying saucer” for an unidentified flying object was to describe a probable meteor that fell over Texas and Oklahoma on June 17, 1930. The highly publicized sighting by Kenneth Arnold on June 24, 1947, resulted in the popularity of the term by U.S. newspapers.
Famous UFO sightings include the 1947 Roswell incident in New Mexico. The 1961 case of Betty and Barney Hill, as well as the 1957 case of Antônio Vilas-Boas, were among the first alien abduction stories to receive widespread attention.
The United States Air Force facility commonly known as Area 51 is a highly classified remote detachment of Edwards Air Force Base, within the Nevada Test and Training Range. The base’s current primary purpose is publicly unknown; however, the intense secrecy surrounding the base, together with reports of unusual phenomena, have led Area 51 to become a focus of modern UFO and other conspiracy theories. Some of the activities mentioned in such theories at Area 51 include the storage, examination, and reverse engineering of crashed alien spacecraft, the study of or meetings with extraterrestrials, and the manufacture of aircraft based on alien technology.