Going back for decades, UFO witnesses have seen unexplained objects leaving and entering the world's oceans, seas, bays, rivers and lakes. In recent years, submarine UFO activity has been observed in the coastal waters off Puerto Rico and Iceland. And in late 1999 1 spoke with the host of a radio show in the Midwestern region of the United States who told me of recent sightings of UFOs seen entering and leaving the waters of Lake Erie. In light of observations such as these, I hypothesize that at least some of the observed submarine UFO activity could be related to clandestine sub-sea (or sub-lacustrine) bases made by terrestrial humans. This is a possibility that I believe researchers should consider when carrying out their investigations. Just because a phenomenon, or suite of related phenomena, are highly strange or bizarre, does not mean of itself that they must necessarily be alien, or extraterrestrial. Indeed, from published accounts in the popular aerospace press of recent years, it is clear that the American military-industrial complex has covertly developed a new generation of technology that has been carefully kept out of the public eye.