Grüner Sea
Grüner Sea is in the Hochschwab Mountains which is in the Northern Limestone Alps of Austria. The range itself is in Styria. The lake is also called the "green like" because of its striking emerald green water, but that's not the only thing that is striking!
In the winter, the lake is only 3-7 feet deep at its deepest, and the surrounding area is used as a county park. In the spring, the park fills up with water and creates an "underground" park where people can scuba dive and swim. Benches and tables stay planted in the ground.