Pannonian Basin is a vast lowland between the Alps, Carpathians, and Dinarides. It covers most of Hungary and parts of all neighboring countries, plus a piece of the Czech Republic (Lower Moravian Valley). 1-10 million years ago, it formed the bottom of the Pannonian Sea. It is drained by the Danube, which broke through the Carpathian barrier at the Iron Gates. Most of the area is around 100 m above sea level, but solitary island mountains or highlands rise in places. Notable hills are found in the Transdanubian Mountains between Lake Balaton and Budapest, featuring limestone massifs and former volcanoes, and in the Pannonian Island Highlands on the southern edge of the basin, in Croatia and Serbia. The highest mountain is the Croatian Ivanščica (1061 m), but the most prominent is Sljeme (1033 m), also in Croatia. The Pannonian Basin contains 4 capital cities - Vienna, Bratislava, Budapest, and Belgrade, all on the banks of the Danube.