Sea Currents (海流 Kairyū?) are forces of major importance in the World. A Sea Current is a directed movement that determines the way water acts on a sea or ocean. Since the world is predominantly covered with water, currents are used to propel ships to certain destinations at greater speed. Currents are usually created by changes of temperature in the water, differences in salinity levels (the concentration of salt), tides, wind and the continuous break and mixing of waves, though there are some exceptions. In the World of One Piece it is even possible to use currents created by massive sea life. The only place where one cannot find sea currents are in the Calm Belts. The Grand Line itself is thought to be one enormous constantly changing current that separates the seas.