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UFO was, during the 20th and 21st centuries, a human designation standing for Unidentified Flying Object. Many UFOs were believed by some of the general public, such as Eugene Jones and Maxwell Edison, to be alien spacecraft. Particularly in the 1950s and into the 1970s, the public often reported UFOs as flying saucers, crewed by "Martians".
References and appearances
In 1957, the Sixth Doctor and Melanie Bush investigated UFO sightings in the proximity of Hammerston, Louisiana and became involved in the production of the film Swamp of Horrors after attacking an actor in a rubber alien suit whom they mistook for a genuine alien. The Doctor, who used his pseudonym "Dr. John Smith", and Mel were assumed to be uncredited actors playing fictional characters by film critics. (PROSE: Swamp of Horrors (1957) - Viewing Notes)
In 2007, the Tenth Doctor and Rose Tyler went undercover at Deffry Vale High School after Mickey Smith and locals of the borough first noticed UFO activity - "lights in the sky" - around the area, later shown to be of Krillitane origin. (WC: Tardisode 4, TV: School Reunion)
In 2400, a crewman on an observation satellite orbiting Earth reported seeing hundreds of flying saucers (actually Daleks on transolar disks) on his scanner. A physician told him that flying saucers were a myth of the 20th century and advised sick leave. (COMIC: Invasion of the Daleks)