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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.
Type of CurrentsEdit
Surface currents are currents found in the warmer waters closer to the surface, and are quite fast moving.
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 are natural forces of disasters that form a sinkhole in the water which can draw nearly anything into its depths. In One Piece there are two ways for a whirlpool 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 Hat Pirates were descending to Fishman Island.
Reverse Mountain (リヴァース・マウンテン Rivāsu Maunten?) is a place where the currents of the four Blue Seas converge and flow giving an entrance to 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 four Blue Seas, but not colder than 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 Edit
The Knock Up Stream (突き上げる海流（ノックアップ ストリーム） Nokku Appu Sutorīmu?) is considered as a natural disaster by most, but some like Nami describe it as an extremely 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 pressure gets too high, causing the sea to explode upwards. The torrent may vary in location and power from time to time, but was once powerful enough to send a portion of an island up into the Sky Island.
On Long Ring Long Land, around one of the ten islands that make up the whole, is a large whirlpool. The whirlpool forms between a 'forest' of Long Corals and the Long Gap, and as its name suggests, it is a long oval shaped whirlpool. This Whirlpool creates a series of currents amongst the Long Coral forest. If one enters the forest through the wrong current, they will find themselves being turned back, though the correct one will accelerate them towards the Whirlpool.
The Tarai Current (タライ海流 Tarai Kairyū?, literally translated as "Tub Current" or "Basin Current"; called Tub Current in the VIZ and FUNimation subs), 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. Marines can travel peacefully through this current coating the hulls of their ships with Seastone.
The Serpent Currents (遊蛇海流 Yūda Kairyū?), also known as the Sea Snake Currents in the FUNimation dub, are found on the way to Fishman Island, somewhere between the Florian Triangle and the Red Line. These are 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.
The Downward Plume is described as a massive underwater waterfall, the only one seen in the series is located near a Winter Island that forms part of the Red Line. It goes under the Red Line and is used to get to Fishman Island more quickly. It functions slightly similarly to Reverse Mountain although in the opposite direction, the cold water coming into contact with the Red Line is blasted down towards the ocean floor, moving hot water upwards. A "plume" is the hydrodynamics term for a column of fluid (gas, liquid, plasma, etc.) moving through another and, usually, following its flow. Attaining to the explanation on the series and the scientific term, this is where the "plume" of the name is taken from.
White Strom (白い竜（ホワイトストローム） Howaito Sutorōmu?, "White Dragon") is a giant white whirlpool that appears without any warning on the sea floor, usually on the Grand Line. It said that ships that get caught up in this stream are usually found after days without the crew on board. The name is a portmanteau of the English words "white" and "maelstrom".
A common phenomenon in the New World, a Sea Slope (海坂 Unasaka?) is a current that goes 'downhill' in the open sea. Ships make great time if they were to go 'downhill', though presumably go much slower if they were to go 'uphill'.