‘While this notion may sound unlikely on first hearing, many observers are persuaded that it is at least conceivable,’ the team wrote in their new study, ‘not least because whole swathes of our planet remain virtually unexplored and uncharted.’

With 80 percent of our oceans unmapped, and still revealing ancient mysteries like Yonaguni Jima, the ‘Japanese Atlantis,’ not to mention unexplored caves and the dark side of the moon, they argue there’s plenty of space for a ‘stealth’ civilization.

The researchers described, as one example of a potentially hiding advanced species, a mᴀssive submerged stone structure discovered by a diver off the coast of Yonaguni Jima in Japan that some marine geologists argue are might be the remnants of a ‘Japanese Atlantis’

‘If another intelligent species had inhabited Earth (or Mars) long before Homo sapiens, it is possible that we could have no idea,’ the trio notes in their article, which is set to appear in the journal Philosophy and Cosmology this June.

The work is a collaboration between scientists who have independently tried to make sure that all viable theories are considered for the UFO mystery, which has seen a major effort effort for government declassification in recent months.

Congress, the Pentagon and others in government now more commonly refer to UFOs as UAP for ‘unidentified aerial (or anomalous) phenomena.’

Multiple regions on Earth and near Earth were cited in the new study as worthwhile candidates for investigating the chances of a ‘cryptoterrestrial’ species’ secret base.

One region, dubbed the ‘Alaskan Triangle,’ is a remote and sparsely populated area between the cities of Anchorage, Juneau and Utqiagvik that the authors described as  ‘a prominent ‘H๏τspot’ for UAP [UFOs], as well as other oddities.’

The triangle, they noted, has been ground zero for over 20,000 unexplained disappearances since the 1970s, above and beyond its penchant for UFO sightings.

Luis Guerra, a resident in the central Mexico city of Atlixco, pH๏τographed this image of an apparent UFO above the Popocatépetl volcano in Mexico. Sightings near this and other volcanos have fueled speculation that the UFOs might come from a hidden underground base

More concretely, the researchers pointed to a series of intriguing archeological finds that suggest the existence of ancient civilizations that would not only predate any known advances species, but may still exist in hiding.

They described a mᴀssive submerged stone structure discovered by a diver off the coast of Yonaguni Jima in Japan that some marine geologists argue are might be the remnants of a 5000-year-old pyramid from a Japanese Atlantis.

Habitable regions underground also remain to be explored, some with the right conditions to support life.

‘The internal structure of Earth is still mostly a mystery,’ the team wrote.

‘For instance, it was recently found that the mantle transition zone (255 to 410 miles underground) acts as a large reservoir of water.’

They speculated that it is entirely possible that hundreds of miles below humanity ‘another hominid species, or even a branch of Homo sapiens, could have adapted to live underground,’ although it would likely scarcely resemble us.

Stunning pH๏τos taken by Luis Guerra, a resident in the central Mexico city of Atlixco, last year, have fueled similar speculation of a hidden UFO base underneath the country’s Popocatépetl volcano, which has become a UFO H๏τ spot.

Other major candidates, the researchers put forward, include long-term bases deep underwater in ocean trenches or on the dark side of the moon, whether built by an ancient advanced terrestrial race or our long-term extraterrestrial co-inhabitants.

Other major candidates, the researchers put forward, include long-term bases deep underwater in ocean trenches or on the dark side of the moon, whether built by an ancient advanced terrestrial race or our long-term extraterrestrial co-inhabitants

While researchers working with Nasa’s LROC moon-mapping mission have debunked the theory that this lunar image captured a 3.5-mile (5.6km) ‘alien base’ spire, much of the dark side of the moon remains unexplored and may yet hold such a discovery