The Underwater World involves a large group of pedestrians dancing under the surface of the water in a circular formation, with animations similar to the ones seen in the mission Life's a Beach.
This mystic place can typically be found at the outer edges of the state of San Andreas. Very few valid encounters were reported for this myth before a mod was created to simulate its effects, leading many players to consider its status to be unlikely. The fact that CJ can be seen in an above-ground animation stance in the few screenshots that exist of the supposed event also add serious doubt to its existence in an unmodified game, but there is a glitch, which can make those effects, even allowing the player to fly a cheated Hydra under the water.
This myth could be a glitch.