The Mermaid Tunnel is a location in Endless Ocean that can be found in south-eastern Manoa Lai at coordinates L-7/8. It leads to the Great Aqua Cave, a series of tunnels and caves that link together to form an underwater castle-like structure, despite having little to no evidence of human interference with its formation. In the European release of Endless Ocean, the Tunnel is known as the Mermaid's Grotto.
This area is home to a Green Sea Turtle that patrols around the entrance, a Leopard Whipray on the seafloor, and groups of smaller fish, including Bower's Parrotfish and Green Chromis. A large amount of branching soft pink corals can be found in this area, large populations of which can also be found in areas like Mahla Baoh's Jaw and Between Heaven and Earth.Nearby, the Ancestral Reef landmark can be found, if the player swims to the west and just barely north of the Tunnel.