Siljan, in Dalarna in central Sweden, is Sweden’s sixth largest lake. The cumulative area of Siljan and the adjacent, smaller lakes Orsasjön and Insjön is 354 km2 (137 sq mi). Siljan reaches a maximum depth of 120 m (390 ft), and its surface is situated 161 m (528 ft) above sea level. The largest town on its shore is Mora.
The lake is located around the southwestern perimeter of the Siljan Ring (Swedish: Siljansringen), a circular geological formation which was formed 377 million years ago in the Devonian by a major meteorite impact. The original crater, now mostly eroded, is estimated to have been about 52 km (32 mi) in diameter and is the largest known impact crater in Europe (excluding Russia). The Cambrian, Ordovician and Silurian sedimentary rocks deformed by the impact are rich in fossils.
Some people suspect that there might be oil in the area, but drilling has been unsuccessful so far. There are large deposits of lead and zinc in the Boda area.