A maelstrom (or malström/malstrøm in the Scandinavian languages) is a very powerful whirlpool. The Nordic word was introduced into English by Edgar Allan Poe in his story "A Descent into the Maelström" in 1841.
In turn, the Nordic word may have been borrowed from the Dutch maelstrom (modern spelling maalstroom, meaning literally 'crushing current') and malström in Swedish is the combination of mala, to grind, and ström, meaning current. One can find just such a maelstrom in Saltstraumen sound in Norway.
Located in Nordland, 30 km east of the city of Bodø, Norway in a narrow channel -- connecting the outer Saltfjord with the large Skjerstadfjord -- is the strongest tidal current in the world. Up to 400 million m³ (tonnes) of seawater force their way through a three kilometer long and only 150 meter wide strait every six hours, with water speeds reaching 37 kilometers per hour. Vortices up to ten meters in diameter and five meters deep at the eye of the vortex are formed when the current is at its strongest.
Saltstraumen has existed for over two thousand years. Before that the area was different due to the pressure from the glaciers that had covered Norway for tens of thousands of years. The current is created when the tide tries to fill the Skjerstad fjord. The height difference between the sea level and the fjord inside can be up to one meter and the speed of the current trying to even out that difference can be up to 40 km per hour or 25 miles per hour. When the current turns, there is a period when the sound is navigable. During these times the current is almost calm and one would never suspect it to be the site of one of the world's greatest maelstrom's.
Obscura Day location: April 9, 2011.