Cryptoterrestrial hypothesis

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".

Source: Wikipedia — Cryptoterrestrial hypothesis (CC BY-SA 4.0)