Torghatten

Torghatten is a 258-meter mountain on the island of Torget in Brønnøy Municipality, Nordland county, Norway, known for a natural tunnel that pierces straight through it, about 166 meters long, averaging 18 meters wide and 41 meters high, with its midpoint sitting roughly 115 meters above current sea level. The mountain is built from foliated granite of Proterozoic age, with its internal layering standing nearly vertical, creating planes of weakness that made the rock more vulnerable to erosion along that path. Local legend attributes the hole to a troll's arrow, but geological study by researchers Møller and Fredriksen concluded the tunnel is a polygenetic landform shaped by several different processes across at least two glacial cycles, including meltwater draining beneath the ice, direct glacial action, and wave erosion during periods of higher sea level. Ice roughly a kilometer thick covered the area at the peak of the last glaciation, around 20,000 to 22,000 years ago, and as the ice retreated about 12,000 years ago, meltwater moving beneath the glacier is thought to have opened the initial passage that later erosion enlarged. This entity documents Torghatten specifically as a geological formation, a natural tunnel produced by combined glacial and marine erosion along a structural weakness in granite bedrock.