We know that after the Great Cataclysm, water flooded into the Pit and destroyed the prison, the the inmates went into the surrounding ocean. What we don't know is where that is in the MU. My theory is that it's located on one of the large islands in Mata Nui's feet. It would be far away from most civilizations, and they had east access to the ocean. Also, Voya Nui was far south of Mata Nui (the island), which covered Mata Nui's face, and since Voya Nui was floating above the new Pit, it can't have been far from the old one. My theory is that it's in his left foot, but that's just something I've always thought for no real reason...