What if — stick with us here — an unknown technological civilization is hiding right here on Earth, sheltering in bases deep underground and possibly even emerging with UFOs or disguised as everyday humans?

In a new paper that's bound to raise eyebrows in the scientific community, a team of researchers from Harvard and Montana Technological University speculates that sightings of "Unidentified Anomalous Phemonemona" (UAP) —  bureaucracy-speak for UFOs, basically — "may reflect activities of intelligent beings concealed in stealth here on Earth (e.g., underground), and/or its near environs (e.g., the Moon), and/or even 'walking among us' (e.g., passing as humans)."

Yes, that's a direct quote from the paper. Needless to say, the researchers admit, this idea of hidden "crypoterrestrials" is a highly exotic hypothesis that's "likely to be regarded skeptically by most scientists." Nonetheless, they argue, the theory "deserves genuine consideration in a spirit of epistemic humility and openness."

The interest in unexplained sightings of UFOs by military personnel has grown considerably over the past decade or so. This attention grew to a peak last summer, when former Air Force intelligence officer and whistleblower David Grusch testified in front of Congress, claiming that the US had already recovered alien spacecraft as part of a decades-long UFO retrieval program.

Even NASA has opened its doors for researchers to explore mysterious, high-speed objects that have been spotted by military pilots over the years.

But several Pentagon reports later, we have yet to find any evidence of extraterrestrial life.

That hasn't dissuaded these Harvard researchers, though. In the paper, they suggest a range of possibilities, each more outlandish than the next.

First is that a "remnant form" of an ancient, highly advanced human civilization is still hanging around, observing us. Second is that an intelligent species evolved independently of humans in the distant past, possibly from "intelligent dinosaurs," and is now hiding their presence from us. Third is that these hidden occupants of Earth traveled here from another planet or time period. And fourth — please keep a straight face, everybody — is that these unknown inhabitants of Earth are "less technological than magical," which the researchers liken to "earthbound angels."

UFO sightings of "craft and other phenomena (e.g., 'orbs') appearing to enter/exit potential underground access points, like volcanoes," they write, could be evidence that these cryptoterrestrials may not be drawn to these spots, but actually reside in underground or underwater bases.

The paper quotes former House Representative Mike Gallagher, who suggested last year that one explanation for the UFO sightings might be "an ancient civilization that’s just been hiding here, for all this time, and is suddenly showing itself right now," following Grusch's testimony.

The researchers didn't stop there, even suggesting that these cryptoterrestrials may take on different, non-human primate or even reptile forms.

Beyond residing deep underground, they even speculate that this mysterious species could even be concealing themselves on the Moon or have mastered the art of blending in as human beings, a folk theory that has inspired countless works of science fiction.

Another explanation, as put forward by controversial Harvard astrophysicist Avi Loeb, suggests that other ancient civilizations may have lived on "planets like Mars or Earth" but a "billion years apart and hence were not aware of each other."

Of course, these are all "far-fetched" hypotheses, as the scientists admit, and deserve to be regarded with plenty of skepticism.

"We entertain them here because some aspects of UAP are strange enough that they seem to call for unconventional explanations," the paper reads.

"It may be exceedingly improbable, but hopefully this paper has shown it should nevertheless be kept on the table as we seek to understand the ongoing empirical mystery of UAP," the researchers conclude.

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