Cottbuser Ostsee

The Cottbuser Ostsee (Cottbus Eastern Lake; Chóśebuski pódzajtšny jazor) is an artificial lake under development on the grounds of the former open-pit lignite mine de:Tagebau Cottbus Nord near Cottbus, Brandenburg, Germany.

Dimensions
When complete, it is to cover a surface area of 19 km², making it one of the biggest artificial lakes in the country. At a maximum depth of 30 m and an average depth of between 2.5 m and 3 m the lake is to have a total water volume of 0.150 km3 once flooding is complete. Until the lake is complete, the biggest artificial lake in Germany by surface area (likewise created by conversion of a former lignite mine) is Geiseltalsee which covers some 1840 ha. However, the water volume of Geiseltalsee is almost three times larger at 0.423 km3. For comparison, Germany's largest lake, Lake Constance, covers 536 km2 at a depth of up to 251 m and contains some 48 km3 of water. The largest reservoir (i.e. an artificial lake created via a dam) in Germany by surface area is Forggensee in Bavaria with a surface area of 15.2 km2 and a maximum water volume of 0.168 km3. However, this lake is reduced to a "rump" of 3.2 km2 surface area in winter.

History
The lake covers what used to be the open pit lignite mine "Tagebau Cotbus Nord" which produced a total of 220 Mt of coal during its operation from 1981 to 2015. The last coal was mined in December 2015 and subsequently work began on converting the mine into a lake. Flooding started in 2019 but had to be interrupted several times due to low water levels in the nearby Spree River. The lake is to become part of the Lusatian Lake District, a chain of artificial lakes mostly the result of open-pit lignite mining.

Name
The name - besides referring to the location of the lake to the East of central Cottbus - can be interpreted as a pun on the German name for the Baltic Sea, which is called "Ostsee" in German, the two words only being distinguished by their grammatical gender: "See" meaning "sea" in German is grammatically feminine, whereas "See" meaning "lake" in German is grammatically masculine.