Gruner Sea(Green Lake) is a lake in Styria, Austria in a village named Tragob. The lake is surrounded by the Hochschwab mountains and forests.
The clean and clear water comes from the snowmelt from the karst mountains and has a temperature of 6–7 °C. During the winter, the lake is only 1–2 m deep and the surrounding area is used as a county park. When the temperature rises and the snow melts, the basin of land below the mountains fills with water.