The Cryptoterrestrial Hypothesis: The Idea That UFOs Come From Hidden Earthlings

The cryptoterrestrial hypothesis is the proposal that some reports of UFOs (unidentified flying objects), now often called UAP (unidentified anomalous phenomena), might be explained by an intelligent, technologically capable civilization that lives hidden here on Earth, rather than one that travels from another planet. Depending on who is telling it, the hidden beings might live underground, under the oceans, on or near the Moon, or even among us in disguise. They might be a surviving branch of humanity, an unknown species that evolved alongside us, visitors who arrived long ago and stayed, or something stranger, closer to the fairies and elves of folklore.

The word "cryptoterrestrial" combines the Greek kryptos, "hidden", with "terrestrial", "of the Earth". The term was popularised in the 2000s by the American writer Mac Tonnies, whose book The Cryptoterrestrials was published after his death in 2010. The idea drew a burst of public attention in June 2024, when a speculative paper by Tim Lomas and Brendan Case of Harvard University's Human Flourishing Program and Michael P. Masters of Montana Technological University argued that scientists should keep the possibility open. Headlines announced that "Harvard researchers" thought aliens might be "walking among us".

This article explains where the idea came from, what its proponents actually claim, how it differs from other UFO theories, and, just as importantly, what the evidence does and does not show. In short: the hypothesis is taken seriously by a small number of writers and researchers as a possibility worth considering, but there is no physical evidence that any hidden civilization exists, official investigations have found no evidence of non-human technology, and even its best-known academic advocates describe it as improbable.

Quick summary

The three families of UFO explanation

To understand the cryptoterrestrial idea, it helps to see where it sits among other explanations for UFO reports. The Lomas, Case and Masters paper divides explanations into three broad classes.

1. Conventional terrestrial explanations. UFOs are ordinary things: aircraft, balloons, drones, satellites, rocket launches, birds, planets and stars, atmospheric effects, camera artefacts, misperceptions, hoaxes and secret human military technology. Mac Tonnies called this the "null hypothesis" of UFOs. The overwhelming majority of investigated reports turn out to fall into this class.
2. The extraterrestrial hypothesis. Some UFOs are spacecraft from civilizations on other planets. This is the explanation most familiar from popular culture.
3. Unconventional terrestrial explanations. Some UFOs involve non-human intelligence that is already here, in some sense, without having recently arrived from space. This class is often called the "ultraterrestrial" hypothesis. It includes ideas about beings from other dimensions and time travellers. The cryptoterrestrial hypothesis is the version in which the beings are physically present on Earth, simply hidden.

The cryptoterrestrial idea is therefore not anti-scientific in form: it is a proposed explanation, and it can be compared against the others. The question is whether there is any evidence for it, and whether it explains anything better than its rivals.

Antecedents: hidden worlds in fiction and occultism

The notion of an advanced people hidden beneath or within the Earth is far older than the flying saucer. The Wikipedia article on the hypothesis lists a long line of forerunners.

Fiction

• **Jules Verne's Journey to the Center of the Earth (1864)** imagined a hidden world beneath the surface.
• **The anonymous novel The Coming Race (1871)**, now known to be by Edward Bulwer-Lytton, described a subterranean race of superhumans with mysterious mental powers.
• **William R. Bradshaw's The Goddess of Atvatabar (1892)** pictured an advanced civilization, complete with airships, living inside a hollow Earth.
• **H. G. Wells's The Time Machine (1895)** introduced the Morlocks, a race living underground in the far future.
• **James Hilton's Lost Horizon (1933, filmed in 1937)** gave the world Shangri-La, a hidden valley of wise and long-lived people.
• **The film serial The Phantom Empire (1935)**, starring Gene Autry, involved a singing cowboy discovering an underground civilization.

Occultism and esoteric religion

The founder of Theosophy, Helena Blavatsky, taught that superhuman spiritual "masters" lived hidden in the mountains of Tibet. Occult writers around the turn of the twentieth century elaborated claims about hidden or lost societies in places such as Shambhala, Atlantis, Thule, Hyperborea, Mu and Lemuria, and about a Hollow Earth with openings at the poles. These ideas fed directly into later UFO lore.

The Shaver Mystery

The most direct ancestor of the cryptoterrestrial hypothesis is the "Shaver Mystery" of the 1940s. Richard Sharpe Shaver claimed to have had contact with malevolent subterranean beings who used ancient machinery and flew disc-shaped craft. The science fiction magazine Amazing Stories, edited by Raymond Palmer, published Shaver's claims starting with "I Remember Lemuria" in March 1945, and they caused a sensation among readers.

In June 1947 the private pilot Kenneth Arnold reported seeing a formation of fast-moving objects near Mount Rainier in Washington state, and the press coined the phrase "flying saucer". A wave of similar reports followed. In October 1947 Palmer argued in Amazing Stories that the flying disc wave proved Shaver right. Much later, after Shaver's death in 1975, Palmer admitted that Shaver had spent eight years in a mental institution, being treated for paranoid schizophrenia. Most historians now treat the Shaver material as a product of illness and of Palmer's showmanship, not as evidence of anything beneath the Earth.

Early ufology and the inner Earth

Once flying saucers became a public obsession, a number of writers proposed that they came not from space but from inside or beneath our planet.

• Meade Layne, a Theosophist, argued that saucers came from an "etheric" plane of the Earth.
• Gray Barker, in They Knew Too Much About Flying Saucers (1956), floated an inner-Earth origin.
• Raymond Bernard, in Flying Saucers from the Earth's Interior (1960), popularised the idea of a hollow Earth inhabited by saucer builders.
• Albert K. Bender, in Flying Saucers and the Three Men (1962), explored the same territory.

The political scientist Michael Barkun, in his 2003 study A Culture of Conspiracy, notes that conspiracy literature mixes these ideas in different ways: some writers place alien bases underground while still treating UFOs as extraterrestrial, while others treat the saucer builders as natives of the inner Earth.

Keel, Vallée and the "ultraterrestrials"

In the late 1960s and 1970s the writer John Keel and the computer scientist and ufologist Jacques Vallée moved away from the "nuts and bolts" extraterrestrial view. They noticed that modern UFO and "alien" encounters often resembled older folklore about fairies, demons and other supernatural visitors. Keel used the word "ultraterrestrials" for the beings he thought lay behind these experiences, a category that shaded into the paranormal. Vallée proposed that the phenomenon might act as a kind of "control system" influencing human beliefs over centuries.

A scientific argument against humanoid aliens

Another strand fed the cryptoterrestrial idea indirectly. In 1964 the palaeontologist George Gaylord Simpson published "The Nonprevalence of Humanoids" in the journal Science. He argued that evolution is so contingent that intelligent life elsewhere would be extremely unlikely to look human. Later writers such as Jack Cohen, in Evolving the Alien (2002, with Ian Stewart), made a similar point about the classic big-headed "grey" alien. If reported UFO occupants really are humanoid, some proponents argue, then an earthly origin would be less surprising than an extraterrestrial one. (Simpson and Cohen were not arguing for cryptoterrestrials. They were arguing that human-like aliens are improbable.)

Mac Tonnies and the modern hypothesis

Who he was

Mac Tonnies was born on 20 August 1975 and grew up in Independence, Missouri. He lived in Kansas City, supported himself with retail and office jobs, and became known as a writer and blogger on futurology, transhumanism and the paranormal. His long-running blog Posthuman Blues was named by the Kansas City paper The Pitch as one of the city's best. His published books included a collection of science fiction stories in 1995 and After the Martian Apocalypse (Simon & Schuster, 2004). He co-wrote a play, Doing Time, with the Canadian filmmaker Paul Kimball in 2007, and appeared on radio programmes such as Coast to Coast AM and The Paracast.

Tonnies died of cardiac arrhythmia on 22 October 2009, aged 34. The Cryptoterrestrials: A Meditation on Indigenous Humanoids and the Aliens Among Us was published by Anomalist Books in 2010, with a foreword by the British writer Nick Redfern. A selection from his blog, Posthuman Blues, Vol. I, followed in 2012, introduced by the historian Aaron John Gulyas, who wrote that Tonnies "spoke for his generation with passion, eloquence, and a rare insight."

What Tonnies proposed

Tonnies suggested that the beings associated with UFOs and alien abduction reports might be a secretive, indigenous race, a kind of cryptid, that has lived alongside humanity for as long as humans have existed. On this view, such beings might present themselves to us as extraterrestrials, or as supernatural or occult beings, as a form of disguise.

Tonnies was careful to distinguish his idea from the more mystical ultraterrestrials of Keel and Vallée, which he considered occult. He said the cryptoterrestrial idea was "grounded in a more familiar context", and asked whether the alleged aliens could be "flesh-and-blood human-like creatures that live right here on Earth." His idea shares some features with Vallée's notion of a control system, and the Shaver material is often cited as a forerunner.

It is worth being clear about the spirit of the book. Tonnies called it a "meditation", and Gulyas, who studied Tonnies's work, described the hypothesis as "really more of a thought experiment designed to raise questions". Tonnies was not presenting new evidence. He was asking whether a different framing of old reports might be more coherent than the standard extraterrestrial one.

Who has discussed it since

Writers on UFOs and the paranormal, including Nick Redfern, Jerome Clark and Paul Kimball, have discussed the idea. In 2022 the physicist Harold E. Puthoff, long involved in research on unusual phenomena, published a survey of "ultraterrestrial models" in the Journal of Cosmology, describing the possibility of "sequestered terrestrial cultures... existing alongside us in distinct stealth." In 2024 the philosopher Bernardo Kastrup argued, according to Lomas and colleagues, that a version of the idea was the "most reasonable scenario" for UAP. In 2025 Luis Elizondo, a former Pentagon official who has become a prominent voice on UAP, promoted the hypothesis in the documentary The Age of Disclosure as one possible reading of military UAP reports.

The 2024 paper by Lomas, Case and Masters

Who wrote it

The paper that brought the idea to wide attention was written by three academics:

• Tim Lomas, a psychology research scientist with the Human Flourishing Program at Harvard University;
• Brendan Case, associate director for research of the same program;
• Michael P. Masters, a professor of biological anthropology at Montana Technological University, who has written separately about the idea that UFO occupants might be time-travelling future humans.

Titled "The cryptoterrestrial hypothesis: A case for scientific openness to a concealed earthly explanation for Unidentified Anomalous Phenomena", it was posted on ResearchGate in early June 2024 and was published in the journal Philosophy and Cosmology, volume 33. When it first circulated, Global News reported that it had not yet been peer-reviewed. CBS News noted that the paper was not an official Harvard publication; it carries the Harvard affiliation of two authors. The authors themselves called it, according to CBS, "a speculative thought piece."

Why the authors wrote it

The paper opens with the recent history of official interest in UAP. It describes the publication in 2017 of military videos showing encounters with unidentified objects, the Department of Defense's confirmation that the footage was genuine, and the creation of what became the All-domain Anomaly Resolution Office (AARO). It notes that official US intelligence reports on UAP published between 2021 and 2023 considered more than 800 cases. It cites Sean Kirkpatrick, then AARO's director, as saying in May 2023 that most cases probably had conventional explanations and remained unidentified only for lack of good data, but that around two to five percent were "possibly really anomalous."

The authors also discuss the claims of David Grusch, a former intelligence officer who told a congressional hearing in July 2023 that the US government had a secret program to retrieve and reverse-engineer crashed craft of non-human origin. The paper says that "at the time of writing there is no way to know the validity of his claims", while arguing that various features of the situation mean they should not be dismissed out of hand.

The cryptoterrestrial angle comes from two "empirical mysteries" the authors highlight: reports that some UAP move underwater as well as through the air (sometimes called USOs, unidentified submerged objects), and reports of objects appearing to enter or leave possible underground access points, such as volcanoes. The paper mentions sightings around Popocatépetl in Mexico in 2023, and the oceans' largely unexplored depths. These are reports the authors take as motivating the hypothesis, not as established facts.

What the paper argues

The heart of the paper is an argument for epistemic humility, the idea that we should be modest about what we know. It runs along several lines.

• The geological record is patchy. The authors cite the "Silurian hypothesis" paper by the climate scientist Gavin Schmidt and the astrophysicist Adam Frank (International Journal of Astrobiology, 2019). Schmidt and Frank asked whether an industrial civilization that existed millions of years ago would be detectable in the geological record today, and concluded that its traces could be surprisingly hard to find. Their name for the idea comes from a 1970 Doctor Who story about intelligent reptiles awakened after millions of years. Schmidt and Frank posed this as a question about detection, not as a claim that such a civilization existed; Lomas and colleagues add the further, much bigger step that such a species would have to still be here, hidden.
• Biology may hold surprises. The paper cites the "shadow biosphere" proposal by the physicist Paul Davies and colleagues (2009), the idea that undiscovered life with unusual biochemistry might exist on Earth, and a 2022 estimate that about 20 percent of living mammal species are still undescribed, mostly small rodents and bats.
• Folklore is full of hidden peoples. It surveys traditions of lost lands such as Atlantis, of underground beings such as the serpent-like Nagas of Hindu and Buddhist tradition, and, in detail, of fairies. It cites the Reverend Robert Kirk's 1691 book The Secret Commonwealth of Elves, Fauns, and Fairies, which describes "the subterranean and (for the most part) invisible people" of Scottish folk belief.
• Some places attract recurring legends. The authors mention, for example, Mount Shasta in California, long sacred to local Indigenous peoples and later associated with tales of a hidden advanced community, and speculation about the far side of the Moon.

The authors are explicit about the status of their claim. They write that the hypothesis "carries a very low probability weighting, and even people open to it remain sceptical," and that "we are not arguing that UAP do have a cryptoterrestrial explanation, but simply that they could." They conclude that all versions "are far-fetched on their face" and that the hypothesis "may be exceedingly improbable", but argue it "should nevertheless be kept on the table."

Four versions of the hypothesis

The paper's conclusion sorts its material into four distinct cryptoterrestrial hypotheses (CTHs).

The idea of intelligent dinosaurs in CTH2 refers to a 1982 thought experiment by the palaeontologists Dale Russell and Ron Séguin, who speculated about what a small theropod dinosaur might have looked like had its lineage continued evolving toward a larger brain and upright posture. They called their model the "dinosauroid". It was an exercise in speculative evolution, not a claim that such creatures exist.

The authors note that a similar four-way split appears in a taxonomy of UAP explanations presented in 2023 by Karl Nell, a retired US Army colonel: ancient or proto-human survival, a "breakaway civilization", cryptids or humanoids, and stranded "gods".

The authors' own list of weaknesses

To their credit, Lomas, Case and Masters spell out the main problems with their idea. For the first three versions:

• The physics problem. If cryptoterrestrials are flesh-and-blood beings with advanced technology, they would still have to explain how their reported craft seem to defy physical laws, for example with accelerations that would kill any living occupant. A hidden earthly civilization does not make that problem go away.
• The missing-remains problem. If an advanced civilization existed in the past, why has nothing of it been found, despite extensive archaeological and palaeontological work across the world?
• The silence problem. Why would neighbours who are far more technologically advanced than us remain hidden for so long? The authors note that the extraterrestrial hypothesis can at least claim that visitors have only recently arrived. For a civilization that has lived alongside humans throughout history, the long concealment is harder to explain.

For the "magical" version, the weakness is simply that it is so strange: "belief in (something like) fairies is simply not a live option for many scientists," they write. They suggest, paradoxically, that this strangeness might also be its strength, since it fits the oddest features of some reports, but that is a philosophical move, not a piece of evidence.

How the paper was received

Media coverage in June and July 2024 ranged from neutral explainers to mockery. Outlets including CBS News, Global News and Interesting Engineering ran fairly straight summaries, often with "Harvard" prominently in the headline. Others treated the paper as a joke. Some coverage overstated it as a "Harvard study" or a finding, which the authors' own "speculative thought piece" framing did not support.

Masters told CBS News: "We're not saying this is right, we're not saying that this is absolutely 100% the case," while adding that "it is something that we should all be talking about." He suggested that if reported beings are "ubiquitously described as looking just like us," it "may simply be that they're us", reflecting his separate interest in the idea of time-travelling future humans. Global News noted official responses on related issues: the Pentagon said investigators had not found verifiable information to substantiate claims of secret crash-retrieval programs, and NASA said it had found no credible evidence of extraterrestrial life and no evidence that UAP are extraterrestrial.

What the evidence does and does not show

This is the crucial question for any reader, so it is worth taking slowly.

What there is

1. Unexplained reports. There genuinely are UFO/UAP reports that investigators have not been able to explain. AARO's former director put the share of possibly anomalous cases at a few percent of those reviewed. "Unexplained", however, usually means that there was not enough data to identify the object, not that the object has been shown to be extraordinary.
2. Testimony and claims. There are claims by people such as David Grusch, statements by some members of Congress, and many eyewitness reports, including from military pilots. Testimony can be sincere and still mistaken, and claims made second-hand cannot be checked without the underlying evidence.
3. Gaps in our knowledge. The oceans are largely unexplored, the geological record does lose information over millions of years, and new species are still being discovered. These facts are real and worth knowing.
4. Folklore. Many cultures have stories of hidden or underground peoples. These are real cultural facts.

What there is not

1. No physical evidence of a hidden civilization. No artefacts, structures, remains, DNA, craft or other physical traces of a hidden technological civilization on Earth have been found and verified. No fossil record of a lineage of intelligent dinosaurs or a hidden hominid species exists.
2. No confirmed non-human technology. In March 2024 AARO released Volume I of its Report on the Historical Record of U.S. Government Involvement with Unidentified Anomalous Phenomena, covering 1945 to 2023. It found no evidence that any US government investigation, academic research or official review panel had ever confirmed a UAP sighting as extraterrestrial technology. It reported that it had "found no verifiable evidence" for claims that the government and private companies have had access to or reverse-engineered extraterrestrial technology, and concluded that investigations at all levels found most sightings to be "ordinary objects and phenomena and the result of misidentification." It described the persistence of such claims as largely the result of "circular reporting" among people who believe them.
3. No independent corroboration of the specific claims. Reports of objects entering volcanoes or the sea are, so far, videos and accounts open to ordinary explanations such as aircraft, birds, debris, reflections and camera effects. None has been shown to involve a non-human craft.

Why gaps are not evidence

The cryptoterrestrial case rests heavily on what we do not know. It is true that the geological record is incomplete and that the deep ocean is poorly explored. But "we cannot rule it out" is a very weak kind of support. The same gaps leave room for countless other unlikely possibilities. Scientists usually ask a hypothesis to do more than survive in the gaps: it should make specific predictions that could turn out to be wrong, and it should explain the evidence better than simpler alternatives.

Here the hypothesis struggles. A civilization hidden well enough to leave no trace at all is, almost by definition, hard to test. If no evidence turns up, a proponent can say the beings are simply very good at hiding. That makes the idea hard to falsify, which is one reason many scientists set it aside. Proponents reply that the same was once said of many things later discovered, and that openness costs little. Both points are fair, and they explain why the debate is mainly philosophical.

How the idea could, in principle, be investigated

Although the hypothesis is hard to falsify in its broadest form, its more specific versions do make claims that ordinary science could check. Thinking through what such checks would look like is a good way to see why most researchers remain unconvinced.

• A large hidden population needs energy and leaves waste. Any technological civilization, whatever its biology, uses energy and gives off heat. A community large enough to build and fly advanced craft would be expected to leave some thermal, chemical or seismic signature. Modern satellites measure heat across the Earth's surface, and global networks of seismometers record faint vibrations from earthquakes, mining, explosions and even ocean waves. No unexplained pattern of this kind has been reported.
• Deep underground space is limited. Seismology, which uses the way earthquake waves travel through the planet, shows that the Earth is solid and layered, not hollow. Natural caves and voids do exist, and new ones are still being mapped, but the deep interior is too hot and under too much pressure for open living spaces. A hidden civilization would therefore be confined to the upper crust, where mining, drilling, cave exploration and groundwater studies are carried out all the time.
• The oceans are being mapped. It is true that much of the deep sea floor has not been mapped in fine detail or visited directly. But sonar surveys, submarine traffic, undersea cables, research submersibles and sensor networks are steadily filling in the picture. Specific claims about undersea bases make specific predictions about where something should be found.
• Living beings leave biological traces. If a hidden species of hominid or other intelligent animal existed, it would be expected to leave bones, hair, environmental DNA or other biological traces. The same reasoning applies to claims about cryptids such as Bigfoot, which the 2024 paper also discusses. Despite decades of searching, no such remains have been confirmed for any of these proposed creatures.
• Craft would leave physical evidence. Objects that fly and dive leave radar returns, sonar contacts, debris, and sometimes physical effects on the ground or water. The strongest possible evidence for any non-human-technology hypothesis would be an object or material that independent laboratories could examine. That is exactly what official investigations have said they have not found.

None of these checks can prove that no hidden civilization exists anywhere. A proponent can always suggest that the beings use technology that leaves no trace. But each additional assumption needed to explain away the missing evidence makes the hypothesis less economical. This is the core of the sceptical case: not that the idea is impossible, but that it requires more and more special assumptions to stay consistent with what we observe, while simpler explanations already account for the cases where the facts are known.

A useful example of scepticism from within

One telling detail comes from the 2024 paper itself. In 2023 the Mexican journalist Jaime Maussan presented to Mexico's Congress what he said were non-human "Nasca mummies". In a footnote, the paper explains that its third author, Michael Masters, a biological anthropologist who specialises in hominin comparative anatomy, examined X-ray and CT images of the bodies and found strong signs that they were not authentic, beginning with a lack of bilateral symmetry. Even researchers open to unconventional explanations, in other words, find that specific pieces of claimed evidence often fail close examination.

Weighing the hypotheses

Most scientists conclude that the conventional explanations are by far the most likely for the cases where enough is known, and that the remaining cases are best described as "not yet identified" rather than as evidence for any particular exotic idea. The authors of the 2024 paper themselves rank the cryptoterrestrial hypothesis below the extraterrestrial one. Their argument is not that it is likely, but that it should not be ruled out in advance.

Why the idea appeals

Even if it is unlikely, the cryptoterrestrial hypothesis has a real attraction, and it is worth understanding why.

• It avoids the distance problem. Interstellar travel is extraordinarily difficult. Distances between stars are vast. An earthly civilization does not need to cross them.
• It fits humanoid reports. If evolution elsewhere would rarely produce human-like beings, as Simpson argued, then humanoid "aliens" might more naturally be earthly.
• It connects folklore and modern sightings. Stories of hidden peoples, fairy abductions and lights in the sky go back centuries. The hypothesis offers a single frame for old and new.
• It is a good thought experiment. As Gulyas suggested, the idea is useful for questioning assumptions: why do we assume UFOs, if they are anything unusual, must come from space?

Critics would add that these appeals explain why the idea is attractive as a story, but say little about whether it is true.

The hypothesis in popular culture

Hidden or underground civilizations that might be mistaken for aliens are a recurring theme in fiction, film and music. Examples noted in the Wikipedia article include:

• Gardner Dozois's story "Chains of the Sea" (1973), a science fiction story on related themes.
• **The film The Abyss (1989)**, in which divers encounter an advanced civilization living in the ocean depths.
• **The X-Files episode "Jose Chung's From Outer Space" (1996)**, in which a contactee's message comes from "inner space".
• **Miles Davis's album Agharta (1975)**, whose cover art by Tadanori Yokoo refers to Agartha, a legendary underground kingdom; the sleeve text describes the supermen of Agharta visiting the surface.
• Afrika Bambaataa and WestBam's "Agharta – The City of Shamballa" (1998), released as I.F.O., whose video shows UFOs emerging from a hole at the pole.
• **The Godzilla MonsterVerse films (from 2014)**, in which giant creatures native to Earth live hidden in a "Hollow Earth".
• **Jordan Peele's film Nope (2022)**, in which the apparent UFO turns out to be a living creature, not a ship.
• Marvel Comics, where the Earth-native Inhumans are sometimes mistaken for alien invaders.
• **The film Black Panther (2018)**, in which the hidden nation of Wakanda's advanced aircraft is described by a child as a "Bugatti spaceship".

Common misunderstandings

"Harvard proved aliens live underground." No. Two researchers affiliated with a Harvard program co-wrote a speculative philosophy paper arguing that scientists should be open to the possibility. It presents no new evidence, the authors call the idea improbable, and it is not an official Harvard position.

"It's the same as the Hollow Earth theory." Not exactly. The Hollow Earth is one older idea about where a hidden civilization might live. Modern versions of the cryptoterrestrial hypothesis mostly do not claim the Earth is hollow, which is ruled out by seismology. They suggest caves, deep underground spaces, the ocean, the Moon or concealment among humans.

"It's the same as ancient astronauts." No. The "ancient astronauts" idea promoted by Erich von Däniken and others holds that extraterrestrials visited Earth in prehistory. The cryptoterrestrial idea holds that some intelligence is here now, hidden. The two can overlap in the version where former visitors stayed.

"Unexplained means alien." No. An unexplained report is one that has not been identified, usually because the data are poor. It is not evidence for any particular explanation, earthly or otherwise.

Frequently asked questions

Who came up with the term "cryptoterrestrial"?

The term was popularised by the American writer and blogger Mac Tonnies in the 2000s, through his blog Posthuman Blues and his book The Cryptoterrestrials, published after his death by Anomalist Books in 2010. The underlying idea, of a hidden race behind flying saucers, goes back at least to the Shaver Mystery of the 1940s and to earlier fiction and occult writing.

Is the cryptoterrestrial hypothesis taken seriously by scientists?

Very few scientists consider it likely. A small number of academics, notably Tim Lomas, Brendan Case and Michael Masters in their 2024 paper, argue that it should not be dismissed out of hand. Even they say it carries "a very low probability weighting". Most scientists regard the conventional explanations as far better supported for the cases where enough data exist.

What did the 2024 "Harvard" paper actually claim?

It argued for "scientific openness" to the possibility that some UAP reflect intelligent beings concealed on Earth or nearby, and set out four versions of the idea: surviving ancient humans, a hidden non-human species, aliens or time travellers who settled here, and fairy-like "magical" beings. The authors were explicit that they were not claiming the hypothesis is true, only that it could be.

Is there any evidence for a hidden civilization on Earth?

No verified physical evidence exists. There are unexplained UFO reports, testimony and folklore, but no artefacts, remains, structures or craft attributable to a hidden civilization have been found. The US Department of Defense's AARO reported in 2024 that it found no verifiable evidence of extraterrestrial or non-human technology in government possession.

How is this different from the extraterrestrial hypothesis?

The extraterrestrial hypothesis says UFOs come from other planets. The cryptoterrestrial hypothesis says any non-human intelligence behind UFOs might already live here, hidden. Proponents say this avoids the problem of interstellar travel; critics say it creates new problems, such as why such a civilization has left no trace.

What would count as evidence for it?

Physical evidence that could be independently examined: an artefact, a body, a craft, a structure, or biological material that could not be explained by known human activity or known species, examined and confirmed by multiple independent scientists. So far nothing of the kind has been produced.

Why does Gulyas call it a thought experiment?

Because its main value, as the historian Aaron John Gulyas suggested, is in raising questions about our assumptions, for example why we automatically connect UFOs with outer space, rather than in offering a tested explanation.

Sources

• Cryptoterrestrial UFO hypothesis – Wikipedia
• Lomas, Case & Masters, "The cryptoterrestrial hypothesis: A case for scientific openness to a concealed earthly explanation for Unidentified Anomalous Phenomena" (2024), PDF
• Mac Tonnies – Wikipedia
• The Cryptoterrestrials – Anomalist Books
• CBS News, "Aliens could be 'walking among us' on Earth, Harvard researchers suggest" (July 2024)
• Global News, "Secret UFO civilization, aliens could be here on Earth already: Harvard scientists" (June 2024)
• Space.com, "Pentagon UFO office finds 'no empirical evidence' for alien technology in new report" (March 2024)
• Roll Call, "No evidence of extraterrestrial technology, report finds" (March 2024)
• Aaron John Gulyas, The Chaos Conundrum (2014) – Google Books
• George Gaylord Simpson, "The Nonprevalence of Humanoids", Science (1964) – JSTOR