Grüner See (Green Lake) is a lake in Styria, Austria near the town of Tragöß. The lake is surrounded by the Hochschwab mountains and forests. The name "Green Lake" originated because of its emerald-green water. The clean and clear water comes from the snowmelt from the karst mountains and has a temperature of 6–7 °C (43–45 °F). During the winter, the lake is only 1–2 m deep and the surrounding area is used as a county park. However, during the spring, when the temperature rises and the snow melts, the basin of land below the mountains fills with water. The lake reaches its maximum depth of around 12 m from mid-May to June and is claimed to look the most beautiful at this time. In July, the water begins to recede.
The lake supports a variety of fauna such as snails, water fleas (Daphnia pulex), small crabs, fly larvae, and different species of trout (Salmo). The flora is not abundant because of the rocky bottom of the lake. Furthermore, the lake’s depth is variable since its inflow comes from snowmelt.
The lake is popular among divers who can observe the green meadows in the edge zone of the lake particularly in June when the water is at its highest. A bridge and a bench can also be found underwater.