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In popular culture, the term UFO–or unidentified flying object–refers to a suspected alien spacecraft, although its definition encompasses any unexplained aerial phenomena. UFO sightings have been reported throughout recorded history and in various parts of the world, raising questions about life on other planets and whether extraterrestrials have visited Earth. They became a major subject of interest–and the inspiration behind numerous films and books–following the development of rocketry after World War II. The first well-known UFO sighting occurred in 1947, when businessman Kenneth Arnold claimed to see a group of nine high-speed objects near Mount Rainier in Washington while flying his small plane. Arnold estimated the speed of the crescent-shaped objects as several thousand miles per hour and said they moved “like saucers skipping on water.” In the newspaper report that followed, it was mistakenly stated that the objects were saucer-shaped, hence the term flying saucer.UFO reports have varied widely in reliability, as judged by the number of witnesses, whether the witnesses were independent of each other, the observing conditions , and the direction of sighting. Typically, witnesses who take the trouble to report a sighting consider the object to be of extraterrestrial origin or possibly a military craft but certainly under intelligent control. This inference is usually based on what is perceived as formation flying by sets of objects, unnatural—often sudden—motions, the lack of sound, changes in brightness or colour, and strange shapes.