The Mrežari-area near Jankovac is a 'karst zone' formed about 237–246 million years ago with limestone and dolomite from the Middle Triassic era. These lay on top of older and more robust 'migmatites' and 'gneisses'. This combination created an important 'aquifer', where groundwater is trapped and flows through the carbonate layers. And because these underground water streams dissolve rocks, sinkholes appear, as well as massive underground caves. This area has, by far, the highest density of sinkholes in Papuk with several per square kilometer (km²). Mrežari is a crucial water reservoir due to the karst in the Earth's layers.
Part of: Geosites in Papuk