Great Aqua Cave, otherwise known as the Mermaid's Tunnel (in the European release of the game) is a long tunnel in Endless Ocean. The main rooms/sections in it are White Room, Stalactite Labyrinth, Rim Pool, and others.
The treasures found here are the Patterned Pottery and the Iron Pearl of Manoa Lai.
This is the last room you will encounter. It is in the very back of the tunnel. In here, you will find a false killer whale, a large pearl, black-spotted puffers and more.
This is a large room with stalactites reaching from the ground to the ceiling. Many fish live in here, including John Dory, Longtooth Groupers, and Blue-Spotted Ribbontail Rays.
This is a fairly large room with a small group of west indian manatees and a few other small fish. A waterfall is located directly above it.
This is a small room named for the columns in it. Many shoals of small fish live here. Other than that, there is nothing much of note.