Cryptoterrestrial hypothesis

Non-human intelligences that would already inhabit Earth, hidden, rather than coming from another planet.

The cryptoterrestrial hypothesis proposes that reports of flying saucers or UFOs are evidence of a hidden, Earth-based, technologically advanced civilization. Aaron John Gulyas, a scholar of conspiracy theories, characterized the hypothesis as "really more of a thought experiment designed to raise questions", while others note that "even people open to the cryptoterrestrial hypothesis remain skeptical".