The Plitvice Lakes National Park area is part of the Dinarides karst region, one of the most impressive karst landscapes in the world, marked by specific geological, geomorphological and hydrological properties. The karst relief is primarily tied to carbonate rock (limestone and dolomite rock) due to its strong sensitivity to chemical and mechanical wear, and the influence of tectonics (faults, wrinkles, fissures, etc.). Carbon dioxide enriched water penetrates through fissures in the carbonate rock, dissolving the rock as it flows, creating various surface (such as funnels, depressions, karst fields, towers, columns and more) and subsurface (caves and pits) karst forms.
The park area is dominated by limestone rock of Mesozoic age, with inserts of dolomite rock, and areas of pure dolomite rock. The relationship between the less permeable dolomite rock and the karstified and highly permeable Jurassic and Cretaceous layers of limestone rock have resulted in the present day appearance of this area. The specific hydrogeological properties of the rock have enabled the retention of water on the dolomite rock of Triassic ago, and resulted in the cutting of canyons in the limestone deposits of Cretaceous age. For that reason, the lake system is divided into the Upper and Lower lakes. The Upper lakes are more dominant in the space and volume they occupy, and are formed in dolomite rock, with more indented and gently sloping shorelines than the Lower lakes, and consist of: Prošćansko jezero, Ciginovac, Okrugljak, Batinovac, Veliko jezero, Malo jezero, Vir, Galovac, Milino jezero, Gradinsko jezero, Burgete and Kozjak. The Lower lakes are formed in the narrow limestone canyon with steep banks, and consist of the lakes Milanovac, Gavanovac, Kaluđerovac and Novakovića Brod. Kozjak and Prošćansko jezero are the largest lakes. The formation of these lakes was made possible with the building of the tufa, or travertine, barriers, and the lake system is believed to have been formed 12,000 to 15,000 years ago.
To date, 114 speleological structures have been recorded in the park. This number includes structures areas directly surrounding the park, to about 500 m outside the official park boundaries. In terms of structure, pits are more common than caves in the park. Of the total number, 82 (72%) are pits, while the remaining 32 (28%) are caves, i.e. horizontal structures. Most structures, 91 in total, are small (shallow and shorter than 50 metres). There are 23 structures (20%) that are moderate in size (length or depth ranges from 50 to 500 metres). The total length of structures investigated to date is 1664 m, and total depth is 2251 metres. There are currently no known exceptionally large structures exceeding 500 metres in length or depth.
In the morphological sense, the most interesting structures are the Čudinka pit (-203 m) and the Vršić pit (-154 m, length 110 m). The Čudinka pit is interesting because the entire structure consists of a single spacious vertical shaft, which for many years was considered the deepest in Croatia. In addition to these pits, the caves in the lake zone are also prominent – Mračna špilja (Dark cave) (160 m), Golubnjača (145 m) and Vile Jezerkinje (Lake Fairies’ cave) (104 m), and Golubnjača cave on the Homoljačko polje field (153 m). The caves Vile Jezerkinje, Šupljara and Golubnjača have been specially protected in the category of geomorphological nature monument since 1964. In Rodića cave near Sertić Poljane and in Mračna špilja near the Lower lakes, bones of the cave bear have been found, making these structures of palaeontological significance. The last systematic study of the speleological structures in Plitvice Lakes National park was conducted in the 1960s, and can be considered preliminary.