Tinnsjå ( eng, Lake Tinn), also called ''Tinnsjø'' and ''Tinnsjøen'', is one of the largest

lakes A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...

in Norway, and one of the deepest in Europe. It is located between the municipalities of Tinn and Notodden in Vestfold og Telemark county. At its source in the west, the Måna river flows out of Møsvatn and past Rjukan into Tinnsjå. From the north, the river Mår flows from the Mår, Gøystavatn, and Kalhovdfjorden lakes into Tinnsjå. Tinnsjå is part of the Skiensvassdrag, and drains via the Tinnelva river in the south, down to

Heddalsvatn Heddalsvatnet ( en, Lake Heddal) is a lake in the municipalities Notodden and Sauherad in Vestfold og Telemark, Norway. The main influx comes from the rivers Tinnelva and Heddøla. The lake covers an area of 11.9 km2 or 13.2 km2 according to N ...

. In 1944, during the German occupation of Norway, the

ferry A ferry is a ship, watercraft or amphibious vehicle used to carry passengers, and sometimes vehicles and cargo, across a body of water. A passenger ferry with many stops, such as in Venice, Italy, is sometimes called a water bus or water taxi ...

SF ''Hydro'' was sunk in Tinnsjå by the Norwegian resistance. The Germans were using the ferry to transport a large quantity of heavy water to Germany, where it was to be used for nuclear weapon research. The heavy water had been produced at Vemork, a factory located in Rjukan. The wreck of the ferry was discovered in 1993. In 2004, it was investigated and filmed for an episode of

NOVA A nova (plural novae or novas) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star (hence the name "nova", which is Latin for "new") that slowly fades over weeks or months. Causes of the dramati ...

; heavy water samples were recovered and deuterium isotopic enrichment was confirmed. In 2004 a film crew shooting footage for a new documentary on the heavy water sabotage became aware of an unusual fish, swimming near the lake bottom at a depth of 430 m. Two specimens of the previously unknown fish were captured in April 2005. Analysis revealed the fish to be closely related to Arctic char. The light-colored, translucent fish is up to 15 cm long and lacks a

swim bladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled Organ (anatomy), organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their curren ...

References

Lakes of Vestfold og Telemark Notodden Tinn {{VestfoldTelemark-geo-stub