Lake Pyritsøen and Lake Smaragdsøen are situated close to the coast near Sorthat-Muleby in western Bornholm. The two distinctive woodland lakes are former clay pits from which raw materials were extracted for the Hasle clinker and firebrick factory. When excavation ceased and the pumps were switched off, the deep pits filled with water and became part of the hilly woodland landscape of Hasle Klinkerskov.
Lake Smaragdsøen is the older of the two lakes, while Lake Pyritsøen is larger and dates from the most recent period of clay extraction. A short woodland path connects the lakes, and the area can be explored together with the coastal path, Lertippen and the remains of the former industry.
The area also has a much older history. In 2004, dinosaur footprints approximately 170 million years old were discovered along the coast near Lake Pyritsøen. The tracks date from the Jurassic Period, when Bornholm was the only part of present-day Denmark located above sea level. The discovery helped document that dinosaurs once moved through the landscape that now surrounds Lake Pyritsøen. Footprints from the area can be seen in the exhibitions at NaturBornholm.
Today, the lakes form part of a varied natural area with woodland paths, steep slopes, open stretches of water and only a short distance to the Baltic Sea. Recreational fishing for species including pike and perch is also possible at Lake Pyritsøen.
