In the paper, the researchers discuss four theories of how aliens may be living alongside us, the first of which—the human cryptoterrestrials theory—suggests that there may be an ancient human civilization much more technologically advanced than our own that has somehow survived in secret. The second theory is that there is a technologically advanced non-human civilization that evolved on Earth in secret, created by an ape-like hominid or some "unknown, intelligent dinosaurs."
The third suggests that there are aliens or humans from the future that live on the Earth in secret, and the fourth suggests there are "magical cryptoterrestrials" akin to "earthbound angels," which the researchers compare to "fairies, elves, nymphs."
"The principal weakness of [the final theory], by contrast, is its utter strangeness, particularly for readers schooled to limit themselves to modes of explanation within the bounds of, say, the standard model of physics," the researchers write in the paper. "While belief in extraterrestrials is tenable, belief in (something like) fairies is simply not a live option for many scientists.
"Although this idea is likely to be regarded skeptically by most scientists, such are the nature of some UAP that we argue this possibility should not be summarily dismissed, and instead deserves genuine consideration in a spirit of epistemic humility and openness."