Here’s a concise write-up regarding the search term : Background on the Book Invisible Residents: The Reality of Underwater UFOs is a 1970 book by Scottish-born naturalist and cryptozoologist Ivan T. Sanderson . In it, Sanderson argues that unidentified flying objects (UFOs) are not primarily extraterrestrial in the sense of coming from other planets, but rather originate from a highly advanced, non-human civilization living permanently beneath Earth’s oceans. He coined the term “USOs” (Unidentified Submerged Objects) to describe craft seen transitioning between air and water.