Lake Stymphalia

Lake Stymphalia is located in the north-eastern part of the Peloponnese, in Corinthia, southern Greece.
It is a closed karst basin with a wetland area and an agrarian area. The lake is an important stopover for migratory birds. The size of the lake varies, depending on the season.

Usually this area around the lake is fairly dryerfoot. In certain weather conditions thousands of small green frogs hide in the damp grass from the aquatic snakes. These frogs are invisible in the grass until you almost step on them, at which they jump out of the way, usually landing on another frog which also jumps. The effect is of ripples of frogs travelling outwards with every step. The area is rich in birds, amphibians and plant life. Other wildlife, such as weasels, can also be seen.

Wikipedia