The Škocjanske jame caves have an extremely complex system of cave passages in a total length of 5.8 km. The difference between the lowest and the highest point in the caves is 209 meters. The caves are the biggest and best known natural phenomenon within the classical Karst area. With the shifting of sink holes in the geological past, numerous collapsed dolines have formed at the contact point where flysch meets limestone under the caverns. With their depth of 163 meters, Velika dolina and Mala dolina charm every visitor. The finest view of both dolines with the natural bridge and the cave that separates them is from an observation point. The caves, with an immense underground gorge and halls, are the beginning of the Škocjan underground system. The height of the gorge exceeds 100 meters at several points. The caves probably have the biggest cave hall in Europe, measuring 12,000 square meters (1.2 hectares) in cross section. The Reka river runs underground for almost 40 kilometres, to the sources of the Timav in the Gulf of Trieste.