An unidentified, technologically advanced population could be living secretly on Earth. That startling claim was made in a new paper by researchers at Harvard and Montana Technological University. They speculate that "unidentified anomalous phenomena" (UAP), another term for UFOs, could be living underground, on the moon or even walking among humans. The paper posits the possibility of "cryptoterrestrials" as an explanation for unidentified and unexplainable observations made worldwide each year. Here are the theories proposed in the paper: A "remnant form" of ancient civilization remains on Earth; An intelligent species evolved separately from humans and now stays hidden; Cryptoterrestrials traveled from another time period or planet; The unidentified creatures are of supernatural origin, likened to "earthbound angels." The paper also suggests the idea of cryptoterrestrials living in or under sighting hotspots such as lakes and volcanoes. The researchers propose the influx of sightings in similar areas is due to entry/exit points for hidden societies deep in the Earth. Other possibilities for cryptoterrestrial settlements lie nearby, like on the moon. The paper delves into the government's response to sightings, adding that many believers feel federal agencies are "deliberately downplaying the topic or at least obfuscating the extraordinary nature of many UAP-related events."