Sea Currents

Sea currents (海流) are forces of major importance in the world of One Piece. 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, and differences in salinity (the concentration of salt) levels, though there are some exceptions. The only place where one can't find sea currents are in the Calm Belts. The Grand Line itself is thought to be one enormous current that separates the seas.

Surface Currents
Surface currents are currents found in the warmer waters closer to the surface, and are quite fast moving.

Deep Currents
Deep currents are currents found in the cooler waters closer to the sea bed, and are quite slow moving. It is said that if a ship enters the wrong deep current, it can be trapped in the current for over two thousand years before it can reach the sunlight.

Whirlpools
Whirlpools are natural forces of disasters that form a sinkhole in the water that can draw nearly anything into its depths. In One Piece there are two ways for a whirpool to form.

The first is when a strong current encounters an obstacle, forcing the current to turn in on itself. Such examples are shown with Commander Pudding Pudding went up against the Arlong Pirates, the Long Whirlpool at Long Ring Long Land, and with the whirlpool formed by the Tarai Current in front of the Gates of Justice at Enies Lobby.

The second is caused in cases of extreme differences between hot and cool water, which causes the water to rotate in order to find a constant temperature. Such examples are shown with the Knock Up Stream and with the eruption that took place when the Straw Hats were descending to Fishman Island.

Reverse Mountain
Reverse Mountain is a place where currents of the four blue seas converge and flows into the Grand Line. The currents are extremely powerful as they are able to pull ships up a steep slope. This is due to the fact that the mountain, being a "Winter Island" using Grand Line terminology, is much colder than the surrounding sea in the Blues but not in the Grand Line. However, the mighty currents also cause many ships to crash onto the mountain side due to difficulty in steering into the mountain's opening.

Knock Up Stream
The Knock Up stream is considered as a natural disaster by most, but Nami describes it as a powerful current going straight up. It is created when water gets trapped in an underwater cave, where it is super-heated by lava beneath it, until the pressure gets too much, causing the sea to explode upwards. The torrent may vary from time to time, but at once it was powerful enough to send a portion of an island up into the Sky Island.

Tarai Current
The Tarai Current, or the tub current, is a massive whirlpool that connects the three major World Government facilities. Its flow is controlled by the three Gates of Justice, and will either flow or cease based on how much of any of the three gates are opened. The current goes in a circle within the three gates, and is faster for ships to travel using this current than going around the perimeters of the gates to reach any of the three Government facilities. However, this only works if the Government is willing to open the gates for the said passengers.

Serpent Currents
On the way to Fishman Island, somewhere between the Florian Triangle and the Red Line, there is a series of currents that tend to leap out of the water. They differ between the anime and manga.

In the manga, they form arches and the Straw Hats are briefly seen riding one.

In the anime, they are more like geysers, shooting straight up, but then bending and coming back down. The Straw Hats had to literally battle through them before they could proceed any further.

Downward Plume
Described as a massive underwater waterfall, the only one seen in the series is located near a winter island that makes up part of the Red Line. It is used to get to Fishman Island more quickly. It functions slightly similarly to the reverse mountain although in the opposite direction, the cold water coming into contact with the Red Line is blasted down towards the ocean floor.