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. These areas almost never have ocean currents or winds blowing, and are also nesting grounds for Sea King's. They are very effective barriers for those trying to enter the Grand Line. The Straw Hat Pirates encountered the Sea Kings and were caught on top of one of their heads after the crew inadvertently entered the Calm Belt. The crew and their ship were sneezed away by a larger Sea King, which implies that the Sea Kings 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 ships so Sea Kings won't notice a ship above them.

The island of Amazon Lily is one of the few locations which is known to be within the Calm Belt zone. It's isolation and it's protection by Sea Kings protected the island from intruders as well as prevent anyone outside their own to pass within the island. Silvers Rayleigh is the only known person so far to have reached Amazon Lily by swimming and withoug much hindrance by the Sea Kings.

The Great Gaol, Impel Down, is built in the middle of the Calm Belt, where it is under the seafloor. Because of 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.

Captain "Calico" Yorki of the Rumbar Pirates is said to have fled the Grand Line via the Calm Belt after he and several other crewmates contracted a deadly disease. Their fate is unknown but it is said that they perished before escaping the Calm Belt.