Fish-Man Island

Gyojin Island (Fishman Island) is an island that appears to be underwater.

History
Storywise the concept of Gyojin Island was first mentioned in chapter 69, where Yosaku explain about Jimbei releasing something terrible to East Blue. Arlong and Jinbai had came from Gyojin Island. Sanji then speculate that Gyojin Island may be somewhere in the Grand Line and dreams of meeting beautiful Mermaids.

Oda stated in the SBS that Gyojin Island is half-way across the Grand Line. This is meant in the most literal sense: Gyojin Island is in fact directly underneath the Red Line.

Later after the Enies Lobby Raid, the Log Pose started to point down, Kokoro, who is a Merfolk from Gyojin Island explains that it's an underwater where the Fishman lives. There is an unique way of going down there.