Ever wondered what secrets lie beneath the tranquil surface of Hammer Sø? This largest lake on Bornholm holds a fascinating past. Hammer Sø or Hammer Lake as it’s known in English is Denmark’s only tarn. It’s nestled in a valley in Hammeren the island’s northernmost point. A thin strip of land separates it from Opalsø an artificial lake. Opalsø was created in a granite quarry that closed in 1970.
Hammer Sø’s beauty is undeniable. It stretches about 650 meters long and 150 meters wide. The water reaches a depth of 13 meters. Its surrounding area is a mix of rocky terrain forest and farmland. But Hammer Sø wasn’t always a lake. Long ago it was a sound separating Hammerknuden a prominent headland from the rest of Bornholm. As ocean levels dropped the sound slowly closed. Over many years the openings to the Baltic Sea near Sæne Havn and Sandvig Bay were sealed by exposed granite. Thus the lake was born.
Throughout history many plans were devised to use Hammer Sø. In the 17th century under King Frederik III a naval port was considered. The Dutch engineer Henrik Rüse even drew up plans. His surveys indicated that a harbor was feasible. Bagge Wandel later investigated. His measurements showed that the lake was higher than Sandvig Bay. He determined that the water level would not affect ships if an opening were made. However the abundance of granite under the sand proved an insurmountable obstacle. The naval port was built elsewhere at Christiansø instead.
Another ambitious scheme emerged in 1716. Tsar Peter the Great envisioned a harbor with locks to raise ships up to Hammer Sø’s level. That plan too remained a dream. Even in 1737 and 1809 and again in 1889 proposals for a port resurfaced. In 1889 however it was for an industrial port for the nearby quarry. The idea was abandoned in favor of the harbor at Sæne. Fortunately Hammer Sø remains. Its serene waters a testament to Bornholm’s rich and varied history. Today though driving along the lake’s edge is prohibited. A parking lot west of the lake offers access to the area. You can also explore Opalsø Crystal Lake and Siberia. Each with its unique history and charm. In winter when the lake freezes over locals often transform it into a skating rink. Hammer Sø. Its story is as unique and captivating as the island itself.