Martinsloch, Natural rock arch in the Glarus Alps, Switzerland

Martinsloch is a natural opening in the rock of Grosses Tschingelhorn in the Glar Alps. The hole passes completely through the mountain, allowing light to pass through from one side to the other.

The Martinsloch is a roughly 19-metre-wide, 22-metre-high triangular opening just below the ridge of the Tschingelhörner, at about 2600 metres above sea level. It was formed by geological faults, not by water erosion. Its solar alignment was already recorded in travel accounts: the naturalist H. Besson observed the phenomenon in 1777, and Johann Gottfried Ebel described the sun lighting the Elm church tower on dates of the old calendar.

The name links the opening to Saint Martin, whose feast day falls on 11 November, near the autumn sun event. From Elm the hole is partly hidden by a rock fold in front of it, so only about an eighth of the sun's disc shows through. Travel reports since the 18th century tie the sunrise to the lighting of Elm's church tower.

The site is reached via hiking trails that lead up from the valley below, where you can view the hole and observe light passing through it. The routes require standard mountain walking ability and proper footwear.

On two specific days each year, sunlight shines directly through the hole and creates a bright spot visible from the valley far below. This fleeting occurrence lasts only a few minutes but transforms this geological feature into a rare natural phenomenon to witness.