Grüner See (Green Lake) is a lake in Styria, Austria in a village named Tragöß. The lake is surrounded by the Hochschwab mountains and forests. The name “Green Lake” originated because of its emerald-green water which comes from the snowmelt from the karst mountains.

During the winter, the lake is only 1–2 meters 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 meters (40 ft) from mid-May to June. In July, the water begins to recede. The submerged park is a popular destination for divers to explore the unique environment. [source]