A natural vertical cavern in the slope of Mashuk, with a sulphuric lake at its bottom, called the Proval, is one of the most famous sights of Pyatigorsk. It is mentioned in «A Hero of Our Time» by Lermontov and «The Twelve Chairs» by Ilf and Petrov.

According to Lermontov, all «water society» liked to have a walk to the Proval, although at that time you could only climb up the slope of the mountain to the the edge of the cavern and look into it. The cone of the cavern is 15 meters round, and the height of the cavern is 41 meters. At the bottom of the cavern is a lake with sulphuric water of turquoise color. The lake is 11 meters deep. 

The temperature of water is in the range from 26° to 42° in different seasons of the year. Proval Lake became a real sight in 1858 when a horizontal tunnel had been driven leading to a big apron in front of it. This made it possible to approach the lake and even use it for treatment purposes, although there is a not very pleasant smell of sulfurated hydrogen in the cave.

Eventually, there began the development of the area surrounding Proval Lake. It was there that the most expensive mansions and dachas were located. Later that area of Pyatigorsk was called the Proval town. A beautiful boulevard led from the nearby Academic Gallery to the entrance to the Proval.

Today there are a lot of sanatoriums and health centers on Gagarin Boulevard leading from the center of the city to the Proval. A monument to Ostap Bender offering the tickets to the Proval, as in the novel «Twelve Chairs», is installed near its entrance.