Proval Lake is a karst cave about 18 meters deep, filled with blue hydrogen sulphide water, which has healing properties. It has become a symbol of Pyatigorsk and is known from the novel "12 Chairs" by Ilf and Petrov. Do not expect any particularly pleasant sensations from this place. The smell of hydrogen sulphide is peculiar, it is unlikely to please you. And the lake itself is very, very small, about the size of a garden pond, partially covered with a white film. There are grates installed so that persistent tourists do not fall into this smelly pond. Probably, if you look at this pond from above, the view is more mysterious. The passage to the "lake" is a pedestrian tunnel through the rock.