The Pannonian Basin and the Dinaric Alps, along with the Adriatic Basin, represent major geomorphological parts of Croatia. Lowlands make up the bulk of Croatia, with elevations of less than above sea level recorded in 53.42% of the country. 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. The Dinaric Alps contain the highest mountain in Croatia— Dinara—as well as all other mountains in Croatia higher than . Croatia's Adriatic Sea mainland coast is long, while its 1,246 islands and islets encompass a further of coastline—the most indented coastline in the Mediterranean. Karst topography makes up about half of Croatia and is especially prominent in the Dinaric Alps, as well as throughout the coastal areas and the islands.