Proval Cave (Pyatigorsk)

Proval is a natural sinkhole of karst-tectonic origin on the southern slope of Mount Mashuk in Pyatigorsk. It is an underground cave with a lake, formed by mineral water: as hydrogen sulphide evaporated, it gradually dissolved the limestone rock until the vault collapsed. This created a sinkhole about 15 metres in diameter and 41 metres high, with a grotto and a lake 11 metres deep. The water's turquoise colour comes from its high sulphur content and certain bacteria, and its temperature stays between 26 and 42°C.

Until the mid-19th century the only way down to the lake was along a narrow path between bushes and rocks - this is how Mikhail Lermontov described it in his novella "Princess Mary". On 15 July 1858, funded by the patron P. A. Lazarik, a 43.7-metre tunnel was cut through the rock, opening free passage to the lake.

Proval became known across Russia thanks to Ilf and Petrov's novel "The Twelve Chairs", in which Ostap Bender is amazed that the city still doesn't charge an entrance fee. This fictional episode was played out in reality: from 1928 to 1938 entry really was paid. In 2008 a bronze statue of Bender with a chair was installed at the tunnel entrance, and the entrance itself is flanked by two lion figures, made in the mid-20th century to sketches by the sculptor I. F. Shakhovsky.