Hammersø

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.

Geography
The lake is about 650 m long and 150 m wide, covering an area of 10.3 ha, and reaches a maximum depth of 13 m. Its encatchment area is approximately 133 ha, consisting of about 50% rocks, 25% forest and 25% cultivated land.