The secret sea is an underground sea beneath Deltora. These subterranean waters are located just underneath the land of Deltora, in large caverns connected to one another by tunnels also filled with water. These caverns has seven individual "seas," each located under their respective territory aboveground. These are the Topaz sea, Ruby sea, Opal sea, Lapis Lazuli sea, Emerald sea, Amethyst sea and Diamond sea.
In three of these seas lies a separate island where one of the three Pirran tribes resides. When you leave the secret sea you have to be holding a special rock, or otherwise you will forget that you ever went there. Surrounding these seas is a grey cavern roof that is beneath Deltora'a shores and the Barrier Mountains. Beyond that is an endless black sea from every direction until the edge of the Earth.