The Martinsloch at an altitude of 2,600 metres above sea level in the midst of the Tschingelhörner in Flims is perhaps the most famous rock opening in the UNESCO World Heritage Site Tectonic Arena Sardona. The 22-metre-high, 19-metre-wide, triangular-shaped opening has become famous due to a unique event. Each year, on 12 and 13 March and again on 30 September and 1 October, just before sunrise, the sun bursts through the Martinsloch opening and lands directly on the church in the village of Elm, located just behind the mountain. The spectacle lasts just a few minutes, before the sun disappears behind the Tschingelhörner and then rises.
There are many legends relating to the origins of the famous rock formation. The best-known one, however, is the legend of the shepherd Martin, who tended to his animals on the Elm side of the mountain. One day, a giant from Flims attacked his flock and tried to steal some of his sheep. However, Martin defended his animals valiantly and the giant fled. Martin hurled his staff after the giant, but it missed the giant and struck the Tschingelhörner. A mighty roar and clatter rang out, and a vast sea of rocks crashed down to the valley. Once the dust had settled and calm was restored, a triangular-shaped hole was visible in the rock face, henceforth to be known as the Martinsloch.