Grüner See (Green Lake) is a lake in Styria, Austria near the town of Tragöß. The name 'Green Lake' originated because of its emerald green water. During the winter, the lake is not very 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 Green Lake reaches its maximum depth from mid-May to June and is claimed to look the most beautiful at this time.
(thanks Miss Rare)