An unidentified flying object (UFO) is any airborne object or optical phenomenon that an observer cannot readily identify. Interest in UFOs increased after World War II with the development of rocketry. Some believed that UFOs were evidence of intelligent extraterrestrial life visiting Earth.
The first widely publicized UFO sighting occurred in 1947, when businessman Kenneth Arnold claimed he saw high-speed, crescent-shaped objects. A newspaper misreported that the objects were saucer-shaped, coining the term flying saucer.
From 1952 to 1969 the U.S. Air Force investigated UFO sightings through Project Blue Book. Of 12,618 sightings, 701 remained “unidentified” when the project concluded. The project aimed to apply a methodical approach to the program by documenting subjective experiences.