Keel challenged the extraterrestrial hypothesis, proposing instead that UFOs and related phenomena represent a form of "ultraterrestrial" intelligence that coexists with humanity, often manipulating human perception and belief systems. His work emphasized the psychological and parapsychological aspects of UFO encounters.

Keel worked as a journalist and served in the US Army during the Korean War. He wrote extensively for men's adventure magazines and later focused on anomalous phenomena, publishing numerous books including "The Mothman Prophecies" and "UFOs: Operation Trojan Horse."

Notable works: The Mothman Prophecies, UFOs: Operation Trojan Horse, The Eighth Tower, Our Haunted Planet

Representative statement: Keel argued that UFO phenomena are not necessarily physical spacecraft but may be "windows" into other dimensions or manifestations of a non-human intelligence that has always interacted with humanity.

Keel's work is respected for its depth and originality, though his ultraterrestrial hypothesis remains controversial and is not widely accepted by mainstream science. His focus on the psychosocial aspects of UFO sightings has influenced later researchers examining the intersection of folklore, psychology, and anomalous experiences.

John Keel's books continue to be influential in paranormal research and popular culture. "The Mothman Prophecies" was adapted into a major motion picture, and his theories continue to be discussed in contemporary UAP research circles.