The Skocjan Caves are the most important underground phenomenon in the Karst and in 1986 were entered on UNESCO’s list of natural and cultural world heritage sites. The underground canyon of the river Reka that runs through the caves is so far the largest known in the World;  the river not only deepens its riverbed through erosion, but also by means of corrosion – it dissolves the limestone. The Skocjan Caves consist of eleven caves. The exceptional character of the caves is mainly in its dimensions to which visitors are not left indifferent. The enormous underground spaces achieve their maximum dimension in Martel's Hall, which reached 146 m in height and has a width of 120 m!. A colorful mix of enormous stalactites, collapsed ravines, river canyons, natural bridges and limestone pools are examples of outstanding natural beauty. To stand on the natural bridge with breathtaking sites and views and look at the big river disappearing underground on the bottom of the Great Valley, 160 meters below the surface, is both majestic and frightening. The exceptional volume of the underground canyon is what distinguishes the Škocjan Caves from other caves and places them among the most famous underground features in the world. Moderate walking required on 3km tourist trail; constant temperature in the cave 12°C .