Lake Turnip (Rapne) is a hydrological natural monument of national importance. Located near Slavyansk. The status of a natural monument was assigned by order of the Cabinet of Ministers of Ukraine No. 780-r dated October 14, 1975. The area is 32 hectares.

Lake of karst origin. The depth of the lake is up to 7.5 meters. The lake is salty, which is unusual for a remote from the sea.

Karst cavities were formed after the dissolution of groundwater deposits of rock salt and gypsum, which remained after the drying of the Perm Sea. Over time, karst cavities were filled with salty underground and fresh melt waters.

The water in the lake has medicinal properties. The bottom is covered with deposits of mud, which has healing properties. The mineralization of water in the lake is 15 g/l. The water temperature in summer is +22…+24°С. The brine of the lake is of the sodium chloride type in its chemical composition, it also contains sulfates.