The Green Lake (Grüner See) spends its falls and winters as a mild-mannered lake, with popular hiking trails around its shore. However, when the winter snows melt in the spring, the drain-off flows down the surrounding mountains and into the lake, raising water levels from one or two meters to ten, covering the nearby park with pristine glacier water.
For most of the year, it's a good place for an Alpine hike. In the mountains with crisp air, away from the city, visitors come for a day of walking around the smallish body of water, a mountain meadow, and the forest around it.
During the warmer months, visitors trade in hiking boots for flippers and scuba gear and dive around the suddenly impressive-sized lake. Fish swim around park benches and swimmers float over bridges. The bottom is covered in grass, giving much of the lake a green hue, as well as lending to its name. A guest house near the lake is a popular start point for divers, who flock to this surreal experience of exploring the park lying beneath the gentle waves.
In the spring, the shoreline begins to creep up, rising steadily until just before summer. The water peaks around June as much of the runoff has come down and begins receding in July. Scuba divers from around the world gather in May and June for one of the most unique diving experiences in the world, passing over gravel paths and bushes, frolicking through the seasonal Atlantis-style environment that will soon return to the average, run-of-the-mill park it was last year.
The lake supports some aquatic life, such as small fish and underwater insects, but nothing larger than some trout, so no need to worry about losing a limb to the fearsome grass shark or elude any vicious bench crustacean — just enjoy the outdoors, as it sits underwater.