Fish-Man Island

Fishman Island is an underwater island which dwells 10,000 meters below sea level in a giant hole that goes under the Red Line at the bottom of a deep trench. 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. It is encased in a giant bubble, allowing visitors to breathe.

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.

When the Strawhats arrived at Fishman Island, they were blocked from entering the bubble by the New Fishman Pirates who were unsure whether they should trust Luffy and would only let them pass if Luffy became an underling. To get past them, Nami plans to use the last bit of air left in Thousand Sunny's bubble for a Coup de Burst to shoot them through the bubble.

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.