The Underground Cyclops Monster is a rock-skinned cyclops-like monster who is one of the Underground Monsters that appears in the 2003 TV series episodes Notes from the Underground, Part 2, Notes from the Underground, Part 3 and Return to the Underground. He also appears as an enemy in the Game Boy Advance version of the 2003 video game.
Like Quarry, Razorfist, Stonebiter, King Nail, and other mutants, the Cyclops was originally a human experimented on by the Foot Clan to create subterranean digging creatures in their search for the Utroms. However, he and his brethren resisted the efforts of the Foot to control them, turning on those who had mutated them. Sadly, the Cyclops and many of his fellows soon became completely feral, mindlessly attacking anything and anyone not recognized as one of its own kind. Eventually it and the other mutants were captured by the Entity, who set about restoring them to human form so that they could repopulate and rebuild Atlantis. However, the Turtles managed to rescue him from this fate, only to learn that only the city's Crystal Moon could keep him human. He and the others were forced to remain in the city, and reverted to mutant form when the Crystal Moon fell into the lava surrounding their city. Fortunately, the Turtles returned and were able to restore them to human form permanently.