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The beginning of the trail from Kamenjak, with Goli otok ahead, on the island of Rab (Walk 6)

Croatia is karst country. Karst is formed by the gradual dissolving of the limestone rock by rainwater – or, more specifically, by the combination of rainwater and carbon dioxide from the earth’s atmosphere and the soil, which results in a weak solution of carbonic acid. Over millennia this process gradually enlarges surface drainage holes, as well as horizontal and vertical cracks and fissures in the rock, as the water percolates downwards, creating distinctive surface features such as vertical fluting (karren), solution pans, limestone pavement and sinkholes (doline), while beneath the surface it leads to the formation of an extensive network of caves and underground drainage channels, with all rainwater rapidly disappearing underground.

Collapses in the roof of these subterranean voids leads to the formation of larger sinkholes and cenotes. (Most of the deeper caves and sinkholes in Croatia are on the mainland, and some of these are extremely deep – Lukina jama on Velebit is among the 15 deepest sinkholes in the world.) Polje – large, level-floored depressions up to several kilometres in size, where a thin layer of alluvial soil has gradually accumulated – often form the only suitable areas for growing vegetables and the cultivation of crops.