The Proval Lake is located on the slope of the Mount Mashuk in Pyatigorsk, Russia. It is an underground Sulphur water lake, which lies in a natural cavern in the mountain. It has a diameter of 15 meters and a depth of about 11 meters. As the waters are rich in minerals, particularly Sulphur, there is a distinct smell that surrounds it. The chemical element also gives the lake its beautiful turquoise color.
In the olden days people were lowered into the lake to bathe in its waters. Today swimming is prohibited. At the entrance to the cavern two lion statues stand at guard. The statue of the literary figure Ostap Bender, who charged people for entrance to Proval, can also be seen here.