Siljan Ring, Impact structure in Dalarna County, Sweden.

The Siljan Ring is a roughly 52-kilometre-wide impact structure in Dalarna, central Sweden, the largest in Europe. Its rim forms a ring of lower ground holding several lakes, the largest being Siljan on the south-southwestern edge, with Orsa Lake to the west and Skattungen and Ore on the northeastern margin. Ordovician and Silurian sedimentary rocks, deformed by the impact, outcrop in a ring within older granites.

A meteorite struck this location 377 million years ago during the Late Devonian period, creating one of Europe's largest known impact craters. The massive collision altered rock layers deeply and eventually shaped the geographic features visible today.

The ring's name comes from Lake Siljan, whose shoreline follows the southwestern edge of the crater. The lakes filling the ring's eroded hollows — Siljan, Orsa, Skattungen and Ore — give the landscape its shape, and the dark band of frost-covered forest seen across the basin marks the wooded ground between them.

To explore the region, visitors use marked trails around the lake and bus services from nearby towns. The best season is May through September when paths are clear of snow and daylight hours are long.

Fossilised fungi found deep in the crater's fracture system were dated to about 39 million years old, and 2019 research showed microbes had produced and consumed methane there for at least 80 million years. Two 2025 studies confirmed that microbial methanogenesis is still active in the deep groundwater of the crater rim.