About 54% of the surface of Croatia is covered by carbonatic rocks (limestone, dolomite, or limestone breccia). The karst is part of the so called Dinaric Karst, a huge karst area along the Mediterranean coast from Slovenia to Macedonia.
The limestone is heavyly karstified with numerous big and beatiful caves, dolines, poljes, sinka and resurgences. Most of the karst is bare karst with little earth and vast areas of limestone pavement.
The longest cave is Dulin ponor at Medvedica, which is 16,396m long. The deepest cave is Lukina jama, 1,392m deep and only 1,078m long, which has a single drop of 516m, making it the cave with the longest vertical drop in the world. Slovačka jama, 1,320m deep is the second deepest cave, and both are located at the Mt. Velebit in the Hajdučki i Rožanski Kukovi, an part of the Sjeverni Velebit National Park.