[Amusing Planet] has a lovely set of photographs up of Grüner See: “Grüner See, literally “Green Lake”, is a lake in Styria, Austria, near the town of Tragoss, located at the foot of the snow-capped Hochschwab mountains. During winter, the lake is only 1–2 meter deep and the surrounding area is used as a county park. It is a particular favourite site for hikers. But as the temperature starts rising in spring, the ice and snow on the mountaintops begins to melt and runs down into the basin of land below. The lake swells up to engulf the entire area including the park. During summer, the lake reaches its maximum depth of around 12 meter and is claimed to look the most beautiful at this time.
Here’s a sample photo:
[Amusing Planet]: http://www.amusingplanet.com/2012/08/gruner-see-styria-park-that-turns-into.html