Although the land area of Croatia is not large (56,594 km², 19th in size among the countries of the European Union), it has an extremely varied relief, as it adjoins several large European relief forms. There are three main types of relief in Croatia: lowland Pannonian, mountainous Dinaric and coastal Adriatic.
The lowlands occupy the largest area, with 53% of the territory under 200 m, while 26% of the country is hilly, with peaks between 200 and 500 m, and 21% lies over 500 m above sea level. The lowest areas are in the northeast region, which forms part of the Pannonian Plain, where the alluvial plains of the Sava, Drava and Danube alternate with the cinder plains of Baranja and Srijem.
Further west, isolated, wooded peaks rise up from the plains to a maximum of 1,000 m (Psunj, Papuk and Krndija). Along the edge of the Pannonian Plain is the hilly peri-Pannonian area with a large proportion of highland, sometimes exceeding 1,000 m absolute height (Medvednica, Ivančica, Žumberačka Gora). The transition to the mountainous region is formed by the hills and limestone plateaus of Pokuplje and Kordun. The mountainous region proper includes Gorski Kotar and Lika, with part of the Dinaric highland, basically lying NW-SE, with the highest mountains along the edges (Risnjak, Mala and Velika Kapela, Plješevica, Velebit and Dinara). In Gorski Kotar there is a sharp contrast between the highland area and the deeply hewn river valleys of the Čabranka, Kupa and Dobra. In inland Lika there are extensive karst fields (Plaščansko, Gacko, Ličko and Krbavsko Polje).
The coastal region extends from the mountains. In the north, it includes the Istrian peninsula, while south of Rijeka the coastal belt is narrow, bordered by high mountains (Velebit) on one side and islands on the other. The southern stretch of coastline mostly corresponds to the historically established region of Dalmatia. The main feature of this area is the dominant karst relief. It is characterised by lengthwise zonality and the division into islands, the coastal belt and the hilly hinterland. The coastal zone is proportionally narrow, bordered inland by steep mountain slopes. It is widest and lowest in the flysch zones of Ravni Kotari in the north, in the central Kaštel region, and in the Neretva Delta in the south. In the hinterland is Dalmatinska Zagora, a hilly region with several broad karst fields (Sinjsko and Imotsko).
About half the territory of Croatia is karst land. It is part of a wider region composed of the same material – known as Dinaric karst, named after a Croatian mountain – which continues into Slovenia in the northwest and Bosnia and Herzegovina and Montenegro in the east and southeast. Karst land is characterised by the predominantly underground circulation of water through porous carbonate rock (limestone, dolomite). As the water dissolves calcium carbonate, it creates characteristic surface formations (sinkholes, clefts, valleys, fields) and underground formations (chasms, caves, grottos). About 50 caves deeper than 250 metres have been discovered. Lukina Jama in the Hajdučki Kukovi region, in the Northern Velebit National Park, is among the 20 deepest caves in the world (explored to a depth of 1,431 m).
Rivers arise from powerful sources, and may flow over- or underground before emerging again at ground level in lower lying areas. Submarine fresh water springs known as vrulje are common. The karst land may be forested or completely bare. Within Europe, Croatia is considered to be one of the classic karst countries. Almost all formations have developed, so some Croatian names for particular karst shapes have been adopted in international scientific terminology. The karst region includes the largest reserves of underground drinking water, which require special protection from the dangers of pollution. Since the level of preservation is high, the Croatian karst region is of exceptional natural value on a European level.