The Calm Belts are two areas that are 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. It has been said that The Marines cross the Calm Belts by using Kairoseki lining at the bottom of their ships so Sea Kings will not notice a ship above them.
Notable Places within the Calm Belt
The Great Jailhouse of the Marines, Impel Down, is built in the middle of the Calm Belt, upon the seafloor. Because of the heavy security and Sea Kings, it is almost impossible to break in or out of it.