Lake Thernuvatn has an area of 0,35 km² and is situated 197 m above sea level. Its discharge, Fossa, spills into the bay Thistilfiord.

No road lies to the lake and the walking distance from the main road is 6-7 km. A passable road lies to lake Krokavatn from where the walking distance is shorter. The lake abounds in rather small lake char and is not netted.

The distance from the capital is 614 km by the Whale Bay tunnel and 15 km from Thorshofn.