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The Saltstraumen has the strongest tidal current in the world. Up to 400,000,000 cubic metres (520,000,000 cu yd) of seawater forces its way through a 3-kilometre (1.9 mi) long and 150-metre (490 ft) wide strait every six hours, with water speeds reaching 22 knots (41 km/h; 25 mph). Vortices known as whirlpools or maelstroms up to 10 metres (33 ft) in diameter and 5 metres (16 ft) in depth are formed when the current is at its strongest. The Saltstraumen has existed for about two to three thousand years. Before that, the area was different due to post-glacial rebound. The current is created when the tide tries to fill the Skjerstadfjorden. The height difference between the sea level and the fjord inside can be up to 1 metre (3 ft 3 in). When the current turns, there is a period when the strait is navigable.