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Adamski's best-selling books were in the genre of Ufology. Ufology is the study of UFOs and other unexplained aerial phenomena. Adamski was a leading proponent of the idea that UFOs were extraterrestrial spacecraft and that their occupants were visiting Earth. He wrote several books about his alleged encounters with aliens, including "Flying Saucers Have Landed" (1953) and "Inside the Space Ships" (1955).