Cupp Coutunn/Promeszutochnaya is the longest limestone cave in the (former) Soviet Union. The cave is part of an extensive area of palaeokarst, perhaps of Middle Cretaceous age, rejuvenated by tectonic movement in Neogene times. During the Middle Quaternary the cave was invaded by thermal waters which deposited calcite, fluorite and metallic sulphides. Post-thermal and modern re-working of these deposits has produced a stunningly decorated cave, with a unique mineral assemblage. The quality and unusual nature of these speleothems matches that of any cave in the world. In 1991, the caves were placed on the Global Indicative List of Geological Sites (GILGES) by a working group of the UNESCO World Heritage Convention.