Fish-Man Island

Fishman Island is an underwater island which dwells 10,000 meters under the sea level, in a giant hole that goes under the Red Line. It serves as an underwater gateway to the New World for those who do not wish to cross over the Red Line above. It is the home of the Fishmen and some Merfolk. Though it is literally directly under the main seat of power of the World Government, it was however controlled by Whitebeard. It is also notoriously difficult to reach - with only 30% of the ships surviving the journey.

History
Because it is the only way for any ship to enter the New World, for if people went through Mariejois they would have to leave their ship behind, Fishman Island was often visited by pirates passing through and the Marines chasing after them. When the Great Age of Piracy began, this problem escalated to extreme proportions. With this, many Fishmen and Merfolk were constantly kidnapped and sold off as slaves. However, when Whitebeard declared the island as his territory at some point in time, the problem was solved as attacking the island and its inhabitants would incur Whitebeard's wrath. As a result, the island was indirectly protected and peace was bestowed.

As Whitebeard died during the war, the island has lost its protection, and its fate is unknown at the moment. After parting ways with Luffy, Jinbe waits on this island for Luffy and his crew.

Culture
Fishman Island is said to be an underwater paradise. Because it is underwater, the only way for a conventional ship to get there is to be coated with a special resin found in the Sabaody Archipelago.

Fishman Karate can also be learned here, more specifically in the Fishman Dojo, as revealed by Oda, in an SBS question and then stated by Hatchan. This dojo is the pride of Fishman Island.

Trivia

 * How it is a City located deep underwater, Fishman Island is most likely based of the city of Atlantis.