Grüner See, Karst lake in Tragöß-Sankt Katharein, Austria

Grüner See lies at the foot of the Hochschwab range in Styria, dammed behind a prehistoric rockslide from the Meßnerin's southern flank. Each spring, snowmelt fills the basin to a maximum depth of about 10 to 12 meters, submerging meadows, trails and benches. The water is cold, 5 to 7 °C, and nearly still, with visibility up to 50 meters.

The basin was originally a grazing meadow that became a local recreation area in the 19th century. In 2016, authorities introduced a complete ban on water activities to protect the natural cycle.

The name Grüner See refers to the emerald-green appearance of the water, caused by fine rock dust suspended in it, which reflects the blue-green parts of light. The lake lies in a hollow sheltered from wind, so its surface stays almost wave-free and the visibility reaches up to 50 metres. A wooden plaque by the shore recalls Johann Hafner, found hanged here on 20 October 1951.

Visitors who want to see the flooded landscape should arrive between mid-May and June when snowmelt reaches its peak. Outside these months, the area appears as a dry park with walking trails.

In winter, the water level drops to 1 or 2 meters, and the surrounding area turns back into a walkable park with dry paths. This seasonal transformation makes the lake an unusual natural phenomenon in the Alps.