Lummelunda Cave

The Lummelunda Cave (Swedish: Lummelundagrottan, also known as the Rövarkulan, "The Robbers' Den") is located in a nature reserve at Lummelunda north of Visby on Gotland, Sweden. The explored part of this karst cave is almost 4.5 km (2.8 mi), making it one of the longest caves in Sweden. It is created by the drainage water from the Martebo mire. The water forms a stream with its outlet in the Baltic Sea. In the 15th to 19th centuries, mills and an ironwork were set up by the stream. Although the entrance of the cave has been known for centuries, the main part of it was discovered by three teenage boys during the 1940s–50s. It is visited by around 100,000 people every year and is one of the major tourist attractions on Gotland.