Bjurälven, Nature reserve in Jämtland, Sweden

Bjurälven is a river in Frostviken parish, Strömsund municipality, northern Jämtland, whose water runs underground for up to three kilometres before surfacing at Dolinsjön. The surrounding karst landscape, a nature reserve bordering Korallgrottan, holds over 300 dolines, including the 20-metre-deep, 50-metre-wide Snödolinen, and the dry Grytrännan channel with its many potholes.

Divers from the Swedish Speleological Society began exploring Bjurälven's flooded passages in 2007, starting an annual expedition that has run every year since except 2012. By the 2018 expedition they had surveyed 2,432 metres of connected cave passages, mostly water-filled, making it Sweden's longest underwater cave.

The name Bjurälven comes from the Jämtland word bjur, meaning beaver. Sweden's last beaver was shot here in 1871, and in 1922 the first reintroduction of beavers in Sweden took place at this spot, led by county antiquarian Eric Festin using beavers from Norway.

The area is challenging to access, particularly in winter when snow covers the terrain and temperatures drop sharply. Visitors should come prepared with warm clothing and sturdy footwear suited to difficult ground conditions.

The largest doline, Snödolinen, is 20 metres deep and 50 metres across; the sun never reaches its bottom, so snow lies there all year. The river also cuts a dry former channel, the 100-metre Grytrännan, whose bed holds numerous small potholes.