People you know may be ‘Aliens’ living on Earth says Harvard University
Human cryptoterrestrials: A technologically advanced ancient human civilization that was largely destroyed long ago (e.g., by flood), but continued to exist in remnant form.
Hominid or theropod cryptoterrestrials: A technologically advanced non-human civilization consisting of some terrestrial animal that evolved to live in stealth (e.g., underground), perhaps a hominid, or a species much more distantly related to us (e.g., descendants of unknown, intelligent dinosaurs).
Former extraterrestrial or extratempestrial cryptoterrestrials: Extraterrestrial aliens or our intertemporal descendants who “arrived” on Earth from elsewhere in the cosmos or from the human future, respectively, and concealed themselves in stealth.
Magical Cryptoterrestrials: Entities that are less like homegrown aliens and more like earthbound angels, relating to the world inhabited by humans in ways that (at least from our present perspective) are less technological than magical, who are known in european languages by names like fairies, elves, nymphs, etc.