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.

Sightings of unidentified aerial phenomena increased, and in 1948 the U.S. Air Force began an investigation of these reports called Project Sign. The initial opinion of those involved with the project was that the UFOs were most likely sophisticated Soviet aircraft, although some researchers suggested that they might be spacecraft from other worlds, the so-called extraterrestrial hypothesis (ETH). Within a year, Project Sign was succeeded by Project Grudge, which in 1952 was itself replaced by the longest-lived of the official inquiries into UFOs, Project Blue Book, headquartered at Wright-Patterson Air Force Base in Dayton, Ohio. From 1952 to 1969 Project Blue Book compiled reports of more than 12,000 sightings or events, each of which was ultimately classified as (1) “identified” with a known astronomical, atmospheric, or artificial (human-caused) phenomenon or (2) “unidentified.” The latter category, approximately 6 percent of the total, included cases for which there was insufficient information to make an identification with a known phenomenon.