 Most of the lowlands are found in the northern regions, especially in Slavonia, itself a part of the Pannonian Basin plain.The plains are interspersed with horst and graben structures, believed to have broken the Pliocene Pannonian Sea's surface as islands.The greatest concentration of ground at relatively high elevations is found in the Lika and Gorski Kotar areas in the Dinaric Alps, but high areas are found in all regions of Croatia to some extent.