Hammersø is Bornholm’s largest lake and lies within the prominent fissure valley that separates Hammerknuden from the rest of northern Bornholm. Measuring approximately 650 metres in length, 150–180 metres in width, covering around 10 hectares and reaching depths of up to 13 metres, it is not only the island’s largest freshwater lake — it is also Denmark’s only mountain lake, formed within a rocky landscape shaped by the forces of the Ice Age.
The lake is surrounded by steep rock walls, woodland and clear traces of the fissure valley’s geological development, giving the area its distinctive, rugged character. Nearby lies Opalsøen, a former granite quarry, which together with Hammersø highlights the contrast between the naturally formed mountain lake and the man-made quarry walls in the granite.
Hammersø is also a popular spot for angling. Species such as perch, pike, tench and roach can be found here, and fishing is permitted with a valid licence. The area is easily accessible via trails on Hammerknuden, and the lake forms a natural part of several well-known hiking routes in the region.
