Fish-Man Island

Fishman Island (also known as Merman island) is an island which dwells underwater.

History
Story wise, the concept of Fishman Island was first mentioned in chapter 69, where Yosaku explains about Jimbei releasing Something terrible to East Blue. Arlong and Jimbei had came from Gyojin Island. Sanji then speculated that the island may be somewhere in the Grand Line, as he dreamed of meeting beautiful mermaids.

Fishman Karate is learned here, as revealed by Oda, in an SBS question. In the same SBS, Oda also stated that Fishman Island is half-way across the Grand Line. This is meant in the most literal sense as Fishman 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 mermaid from Fishman Island, explains that it's an underwater island where fishmen and merfolk live. There is a unique way of going down there.

Location
It is located 10,000 meters under Mariejois, in a giant hole that goes under the Red Line, it serves as an underwater gateway to the New World.