The Calm Belts are two areas of ocean that border the north and south of the Grand Line.
The Calm belts are very effective barriers for those trying to enter the Grand Line; these areas are always completely calm, never having any ocean currents or winds blowing, making it impossible for non-self propelled ships to travel through. They are also nesting grounds for the worlds largest Sea Kings.
The Straw Hat Pirates encountered the Sea Kings and were caught on top of one of their heads after the crew inadvertently entered one of the Calm Belts, until the crew and their ship were sneezed away by a larger Sea King.
It was revealed that the Marines are able to safely cross the Calm Belts by using Seastone lining at the bottom of their ships so Sea Kings will not notice the ship above them.
The island of Amazon Lily is one of the few locations which is known to be within one of the Calm Belts. Its location within a Calm Belt has historically guarded the island from intruders. However, this protection has degraded somewhat due to the Marines' newly invented technique of shielding their boats from Sea Kings with Seastone. Silvers Rayleigh is the only known person so far to have reached Amazon Lily by swimming and without much hindrance by the Sea Kings.
Another natural island in the Calm Belt is the island of Rusukaina, which is located northwest of Amazon Lily.
The Great Prison, 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. 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 241 prisoners.
Captain "Calico" Yorki of the Rumbar Pirates is said to have attempted to flee the Grand Line via one of the Calm Belts after he and several other crewmates contracted a deadly disease. Their fate is unknown but it is said that they perished before escaping that Calm Belt.