It's a pretty strange natural phenomenon, but an area located at the foot of the Hochshwab Mountains in the Austrian region of Styria is a part time park, and part time lake.
In the winter months, the park is pretty dry, and provides a good time hiking far from a city. However, in the summer months, it's a 10m deep lake. The reason for this is because in the summer, the snow and ice around the mountains melts and floods the park. The Green Lake, as it is called, attracts divers who are curious to see how the park looks like underwater.