The interdimensional hypothesis is a proposal that unidentified flying object (UFO) sightings are the result of experiencing other "dimensions" that coexist separately alongside our own in contrast with either the extraterrestrial hypothesis that suggests UFO sightings are caused by visitations from outside the Earth or the psychosocial hypothesis that argues UFO sightings are best explained as psychological or social phenomenon.
 The hypothesis has been advanced by ufologists such as Meade Layne, John Keel, J. Allen Hynek, and Jacques Vallée.  Proponents of the interdimensional hypothesis argue that UFOs are a modern manifestation of a phenomenon that has occurred throughout recorded human history, which in prior ages were ascribed to mythological or supernatural creatures. 
 Jeffrey J. Kripal, Chair in Philosophy and Religious Thought at Rice University, writes: "this interdimensional reading, long a staple of Spiritualism through the famous 'fourth dimension', would have a very long life within ufology and is still very much with us today".
 In the 19th century, various spiritualists believed in "other dimensions". During the Summer of 1947, spiritualists adapted the "other dimensions" folklore to explain recent tales of "flying discs".
 In the late 19th century, the metaphysical term "planes" was popularized by H. P. Blavatsky, who propounded a complex cosmology consisting of seven "planes". The term aether ("ether") was adopted from Ancient Greek via Victorian physics that would later be discredited. The term "ether" was then incorporated into the writings of 19th-century occultists.
 The "etheric plane"  and the "etheric body" were introduced into Theosophy by Charles Webster Leadbeater and Annie Besant to represent a hypothetical 'fourth plane', above the "planes" of solids, liquids, and gases.  The term "etheric" was later used by popular occult authors such as Alice Bailey, Rudolf Steiner, and numerous others.
 On July 4, 1947, occultist Meade Layne claimed that flying discs were "etheric".  Layne claimed to be in telepathic communication with "people in the saucers", arguing "it is possible for objects to pass from an etheric to a dense level of matter and will then appear to materialize. They then will return to an etheric conditions". Layne claimed that "These visitors are not excarnate humans but are human beings living in their own world. They come with good intent. They have some idea of experimenting with earth life." The prior year, it had been reported that Layne consulted a medium who relayed communications from a "space ship named Careeta" that came to Earth from 'an unidentified planet'.
 According to one scholar, Layne coined the term "interdimensional hypostasis" to describe the sightings.[additional citation(s) needed] Layne is regarded as the earliest proponent of the interdimensional hypothesis.
 Although the extraterrestrial hypothesis has remained predominant,[additional citation(s) needed] by the 1970s, the interdimensional hypothesis began to be embraced by some UFO enthusiasts. Paranormal author and UFO enthusiast John Keel recalled:
 I abandoned the extraterrestrial hypothesis in 1967 when my own field investigations disclosed an astonishing overlap between psychic phenomena and UFOs... The objects and apparitions do not necessarily originate on another planet and may not even exist as permanent constructions of matter. It is more likely that we see what we want to see and interpret such visions according to our contemporary beliefs.
 In his 1970 book UFOs: Operation Trojan Horse, Keel argued that a non-human or spiritual intelligence source has staged whole events over a long period of time in order to propagate and reinforce certain erroneous belief systems. For example, monsters, ghosts and demons, the fairy faith in Middle Europe, vampire legends, mystery airships in 1897, mystery aeroplanes of the 1930s, mystery helicopters, anomalous creature sightings, poltergeist phenomena, balls of light, and UFOs; Keel conjectured that ultimately all of these anomalies are a cover for the real phenomenon.[additional citation(s) needed]  Keel used the term "ultraterrestrials" to describe UFO occupants he believed to be non-human entities capable of taking on whatever form they want.[additional citation(s) needed]
 J. Allen Hynek was an American astronomer who served as scientific advisor to the U.S. Air Force UFO studies: Project Sign, Project Grudge, and Project Blue Book.  Hynek pioneered the "Close Encounter" classification system;   Hynek had a cameo in Stephen Spielberg's film Close Encounters of the Third Kind.   Jacques Vallée, a student of Hynek's,  served as the inspiration for the French researcher portrayed by François Truffaut in the film.
 In 1975's The Edge of Reality, Vallée and Hynek consider the possibility of what they call "interlocking universes":
 VALEE:  What other wild hypotheses could we make?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." In his 'landmark' 1969 book Passport to Magonia:  On UFOS, Folklore and Parallel Worlds, Vallee argues for a "parallel universe co-existing with our own". 
The idea was reiterated in Vallée's subsequent writings.  Vallée's summarized his objection in his 1990 paper "Five Arguments Against the Extraterrestrial Origin of Unidentified Flying Objects":
 David Grusch, a former member of the UAP Task Force famous for claiming that the US government was covering up evidence for advanced technology, UFOs, and alien pilots, described the idea as a possible alternative explanation to the extraterrestrial UFO hypothesis for his claims, saying "it could be that this is not necessarily extraterrestrial, and it’s actually coming from a higher dimensional physical space that might be co-located 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".
 The 2014 movie Interstellar employs a mix of the interdimensional and the time-traveler hypotheses. The bulk-beings who built the tesseract inside the supermassive black hole Gargantua are later revealed to be future humans who  have evolved to exist in five dimensions.