Since the 1940s, mainstream explanations of inexplicable UFO reports have attributed them to psychological factors. Meanwhile, fringe explanations have argued UFO reports are caused by intelligent beings, perhaps from outer space, an inner Earth, another dimension, or the future.

A more mundane explanation suggests an undiscovered form of primitive atmospheric life, perhaps akin to a 'sky' microbe, could theoretically account for some UFO reports. Meanwhile, many see UFO reports through a religious lens, interpreting UFOs as angelic, demonic, or related to death.

Religious scholar Jeffrey J. Kripal likens the various explanations for UFOs to the 2014 illustration "The Trickster" by Chris Butler. In the work, a 1970s-style "Grey alien" holds three masks: one mask is a 1930s-style "little green man" representing a classic extra-terrestrial, a second mask is an angelic blonde akin to the 1950s Nordic alien, known as Space Brothers or "Etherians". A third mask is a classic red devil with horns. The image was commissioned to be used as cover art for the 2014 reprint of Ufologist Jacques Vallee's classic work Passport to Magonia. Artist Chris Butler reported his validation when author Jacques Vallee opined the "best part of the re-release was probably the cover", which Vallee felt successfully illustrated the book's thesis.

Antecedents

Scholars of folklore note that many 20th-century explanations for UFO reports have antecedents in late 19th-century culture.

Theosophy is a spiritualism-inspired New Age religious tradition founded in 1875 by Russian-American mystic Helena Blavatsky. Blavatsky and her successers described "highly-evolved" superhumans, variously called Ascended Masters, Hyperboreans, Lemurians, Atlanteans, and Aryans. Theosophy held these beings lived in any number of exotic locations, including lost continents, inside a hollow Earth, on other planets, or in a spiritual realm called the Etheric Plane.

In his 1895 novella The Time Machine, H.G. Wells wrote of the Morlocks, a hidden, subterranean race of technological humanoids who feed on helpless surface-dwellers.The Time Machine was noted for its similarity to The Coming Race The 1933 novel Lost Horizon and its 1937 film adaptation depict Shangri-La, a Tibetan paradise inhabited by peaceful, angelic, nearly-immortal people. The 1935 serial The Phantom Empire starred Gene Autry as a singing cowboy who stumbles upon an ancient subterranean civilization living beneath his own ranch.

In the 1940s, when speculation about UFOs first entered mainstream culture, a number of explanations drew upon theosophical or science-fiction concepts. George Adamski reported contact with Nordic aliens or "Space Brothers" who, like Theosophy's higher beings, were benevolent, telepathic, and "Aryan" in appearance. Another theosophist, Meade Layne, argued UFOs were not "space ships" from outerspace but actually "ether ships" from the Etheric Plane -- a spiritual realm described in Theosophy teachings. Still others, including Richard Sharpe Shaver, argued the UFOs came from a hidden terrestrial world: Atlantis, Lemuria, an ocean base, or the inside of a hollow Earth.Boyle: " Shaver wrote of his experiences within the hollow earth where he was witness to an astounding civilization hidden to the surface world. His "elaborate, deeply paranoid cosmology" featured... a plot pulp SF fans would flock to: two distinct races (a la The Time Machine) at war with one another using the technology of the Titans and the Atlanteans. From a scientific perspective, these are the ramblings of a madman. Palmer had to rewrite the story because of its many typograph- ical errors and while "Shaver claimed he was suffering in the hollow earth, he was actually, according to Palmer himself, suffering in a mental hospital." One cannot discount the fact that Shaver was schizophrenic; he spent nearly a decade in mental institutions, including the famous Ypsilanti State Hospital"

Psychological factors

During the 1947 craze, experts in human behavior argued the reports were best explained as a psychological or social phenomenon. The flying disc craze was compared to Scotland's Loch Ness monster, the panic caused by the Orson Welles broadcast of War of the Worlds, and a sea monster panic caused by a US Armed Forces Radio hoax in Japan. Winfred Overholser, nationally renown psychiatrist, described the reports as a "national hysteria".

In the 1970s and 80s, UFO believers began reporting recovered memories of alien abductions. Most scientists and mental health professionals explain these experiences by factors such as suggestibility (e.g. false memory syndrome), sleep paralysis, deception, and psychopathology.

Psychological factors remain the mainstream explanation for otherwise-inexplicable UFO reports. Several UFO proponents have been reported to suffer from severe mental illness, including Richard Shaver and Philip Schneider.

Alien visitors While most fringe theories of UFOs link them to existence of intelligent 'others' from 'elsewhere', there is considerable diversity of speculation as to the origin and nature of said beings. "Disclosure" advocate Luis Elizondo argues UFOs "could be something from outer space, inner space or, frankly, the space in between." Alternatively, filmmaker Steven Spielberg asks "What if [UFOs] are us from 50,000 years in the future?"

Extraterrestrial spaceships

In the 20th century, a variety of voices argued UFO reports were best explained as being physical spacecraft occupied by intelligent alien beings from other planets, or probes designed by extraterrestrials. For much of the 20th century, extraterrestrials were the go-to or default explanation among UFO believers.

On July 7, 1947, original saucer witness Kenneth Arnold raised the topic of possible extraterrestrial origins, both as his opinion and those who had written to him. Some, he said, "suggested the discs were visitations from another planet."Associated Press story, July 7, 1947, e.g., Salt Lake City Deseret News, p. 3, "Author of 'Discs' Story To Seek Proof" By 1949, Donald Keyhoe, a former Major in the US Marines, claimed that elements within the Air Force knew that saucers existed and had concluded they were likely 'inter-planetary'. In 1952, Life Magazine published "Have We Visitors From Space?" which popularized the Extraterrestrial Hypothesis and is thought to have triggered the 1952 UFO flap.

While remaining popular even into the 2020s, UFO believers began to question the extra-terrestrial spaceship explanation by the 1970s. While 1940 and 50s people frequently speculated about life on other planets, by the 1970s, probes to the Moon, Mars, and Venus confirmed those worlds were uninhabitated and inhospitable to life as we know it. While 1940s audiences suggested UFOs might contain explorers from another world doing a planetary survey, by the 1970s, UFO believers like Hynek and Vallee argued the number of UFO sightings were "far more numerous than required for any physical survey of the earth", an argument strengthened by the insight that UFO reports existed long before 1947. Believers argued reports of alien abductions were not consistent with rational, scientifically-minded biological experiments of the kind extra-terrestrial explorers might conduct. They further argued that humanoid aliens evolving independently on another planet was seen as biologically unlikely.Vallée: "Five Arguments Against the Extraterrestrial Origin of Unidentified Flying Objects" Journal of Scientific Exploration, 1990

Inner-Earth inhabitants

During the mid-1940s, an obscure subculture developed around the science fiction magazine Amazing Stories and its tales of Richard Sharpe Shaver, which were presented as a non-fictional account from "a simple man, a worker in metal, employed in a steel mill in Pennsylvania". Since 1945, the magazine had published Shaver's claims that he was in communication with subterranean beings concerned about atomic pollution who piloted disc-shaped craft.

In the October 1947 issue of Amazing Stories, editor Raymond Palmer argued the flying disc flap was proof of Richard Sharpe Shaver's claims. That same issue carried a letter from Shaver in which he argued the truth behind the discs would remain a secret. Shaver wrote:

After Shaver's death in 1975, his editor Raymond Palmer admitted that "Shaver had spent eight years not in the Cavern World, but in a mental institution" being treated for paranoid schizophrenia.

In his 1956 book They Knew Too Much About Flying Saucers, author Gray Barker suggested the saucers might come from an inner Earth, a connection also explored by Albert K. Bender in his 1962 book Flying Saucers and the Three Men. In the 1980s, Rick Doty and John Lear spread stories of malevolent alients in underground bases who were responsible for cattle mutilation and alien abduction. Their claims were echoed by their associates Bill Cooper, Bob Lazar, and Philip Schneider.

In the 2000s, author Mac Tonnies coined the term "crypto-terrestrial" to describe theoretical, hidden, indigenous humanoids. Tonnies compared his "Crypto-terrestrial Hypothesis" with what he termed the Null Hypothesis of UFOs, the idea that "UFOs can be universally ascribed to misidentified natural phenomena and sightings of unconventional earthly aircraft".Tonnies 2010, quoted in Redfern 2010 Aaron John Gulyas, a scholar of conspiracy theories, characterized Tonnies's hypothesis as "really more of a thought experiment designed to raise questions", while others note that "even people open to the cryptoterrestrial hypothesis remain skeptical".

Interdimensional beings

On October 14, 1946, nearly a year before Kenneth Arnold's sighting that coined the term "flying saucers", occultist Meade Layne achieved national notoriety when the wire service carried a story on his claims of a medium who was in telepathic communication with people in a space ship. Layne claimed UFOs came from the Etheric plane, a spiritual realm taken from Theosophy.https://www.newspapers.com/article/the-los-angeles-times-the-ether-ship-mys/206409331/

By the late 1960s, interdimensional theories were gaining in popularity. In his 'landmark' 1969 book Passport to Magonia, Vallée argues for a "parallel universe co-existing with our own". The idea was reiterated in Vallée's subsequent writings. In 1970, author John Keel coined the term "ultraterrestrials" to describe non-human entities linked to UFOs "which exist in a wavelength of energy which we cannot detect". In 1975's The Edge of Reality, J. Allen Hynek considered the possibility of what he called "interlocking universes": HYNEK: There could be other universe with different quantum rules or vibration rates if you want. Our own space-time continuum could be a cross-section through a universe with many more dimensions. ... Think what a hard time you would have convincing an aborigine that right now, through this room, TV pictures are passing! Yet they're here. You have to have a transducer to see them -- namely a TV set. Well, in the same sense there may be interlocking universes right here! We have this idea of space, we always think of another universe being someplace else. It may not. Maybe it's right here." The 2008 action-adventure film Indiana Jones and the Kingdom of the Crystal Skull utilizes the interdimensional hypothesis and describes ancient aliens not flying into space but into ”the space between the spaces".

Time travelers

It has been suggested that UFO reports are caused by humans traveling from the future using advanced technology to somehow "travel backwards in time".

Due to the theoretical contradictions inherent to retrocausality regarding closed timelike curves, researcher Andrew Knight has labeled retrograde time travel as "pseudoscientific", and the time-traveler hypothesis itself has been challenged by skeptics. American astrophysicist Neil deGrasse Tyson has referred to the hypothesis as illogical.