Alnsjøen (also called Alunsjøen) is a lake in Lillomarka in Oslo, Norway.

The lake is drained by the Alna River to the Oslofjord.

The surrounding area was utilized for copper mining during the 18th century.

[1][2] The lake functions as a minor water reservoir for the city of Oslo (the main water reservoir being the lake Maridalsvannet).

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