Tunnsjøen

or is a lake in the municipalities of Røyrvik and Lierne in Trøndelag county, Norway. The 100.18 km2 lake lies just south of the large lake Limingen, and just west of the border with Sweden. It is 358 m above sea level and has a volume of 8.816 km3. The deepest part of the lake is 222 m deep. It is the seventh largest lake in Norway.

Gudfjelløya
The island Gudfjelløya (Tjåehkere) lies in Tunnsjøen lake. Its highest point is 812 m above sea level; which is 454 m higher than the lake. The peak is named: Gudfjellet (in english "God's Mountain"). This makes Gudfjelløya the highest island within a lake in Norway; and in Europe. Its towering higher than the island of Monte Isola within Lake Iseo, in northern Italy. The island is located in the lake's central parts. The area is about 6.1 km2. To the southwest of it, the lake's second largest island is situated. That is: Litlfjelløya (in english: "Little Mountain Island"); also called Reinøya (in english: "Reindeer Island"). Its highest point is 177 m above the lake's water surface.