Calm Belt

The Calm Belts (凪の帯 (カームベルト) Kāmu Beruto) are the areas just north and south of the Grand Line. They first appeared in Chapter 101 and Episode 54. These areas almost never have ocean currents or winds blowing, and are also nesting grounds for Sea Kings. They are very effective barriers for those trying to enter the Grand Line. The same group of Sea Kings catching the Straw Hat Pirates on top of one of their heads again, they were sneezed away by a larger Sea King, which implies that they don't stay in the Calm Belt consistently. However it was revealed that the Marines crossed the Calm Belt by using the Seastone lining at the bottom of the ship where a Sea King won't notice a ship above them.

The island of Amazon Lily is one of the few which is known to be within the Calm Belt zone, with it's isolation and it's protection by Sea Kings which used to protect them from intruders as well as prevent anyone outside their own to pass within the island.

The Great Gaol, Impel Down, is built in the middle of the Calm Belt, where it is under the seafloor. Along with the heavy security and Sea Kings, it is almost impossible to break in or out of it. However, Shiki the Golden Lion managed to escape by cutting off his shackled legs, and more recently Monkey D. Luffy slipped into Impel Down and managed to escape with over 200 pirates.