Hammersø (English: Hammer Lake), or Hammer Sø, is the largest lake on the island of Bornholm and the only tarn in Denmark. It is located in a valley in Hammeren, the northernmost point of the island. A narrow isthmus separates the lake from Opalsø, an artificial lake formed in a granite quarry which was closed in 1970.

The lake is about 650 metres (2,130 ft) long and 150 metres (490 ft) wide, covering an area of 10.3 hectares (25 acres), and reaches a maximum depth of 13 metres (43 ft). Its encatchment area is approximately 133 hectares (330 acres), consisting of about 50% rocks, 25% forest and 25% cultivated land.

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