Hammersø is the largest lake on Bornholm and the only tarn (glacially-scoured lake) in Denmark. It sits in a depression on the Hammeren promontory, surrounded by heathland and birch scrub, with granite outcrops rising at its edges. The water is clean and clear, and in summer the reflections of the sky and heather give it a quietly beautiful character.
The lake is part of the coastal walking circuit from Sandvig. Fishing is possible with a permit, and in winter the lake sometimes freezes solid enough for skating, though you should check conditions locally before venturing out.