The Calm Belts are the areas just north and south of the Grand Line. 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[1]. The same group of Sea Kings catching the Strawhat Pirates on top of one of their heads again (they got away with the same method as last time, pulling the nose hair on the largest one to make it sneeze them away) 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.[2]

Very few islands are found within the Calm Belt zone. However, the island of Amazon Lily is one of the few which is known to be there, with it's isolation and it's protection by Sea Kings used to protect them from intruders as well as prevent anyone outside their own to pass within the island.[3]

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 impossible to break in or out of there by far.

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