Extraterrestrial Hypothesis in U.S. UFO Investigations: From Project Sign and Grudge to Blue Book at Wright-Patterson
To the point
ETH is the idea that some unidentified flying objects come from other worlds, and in the United States it was one possible explanation investigated by Project Sign, then Project Grudge in 1952, and by Project Blue Book (1952–1969) at Wright-Patterson Air Force Base in Dayton, Ohio, as researchers studied sightings of flying saucers and related phenomena, showing that there is still uncertainty about where unidentified aerial phenomena originate.
