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The underground world of karst: Miljacka II


The cave is a valuable part of Croatia’s geoheritage

The entrance into the Miljacka II cave is in the canyon on the right bank of the Krka River, downstream of the Miljacka waterfall, at an elevation of 115 m. It was formed in Promina deposits. This is the longest topographically explored speleological structure in Krka National Park, currently researched to a depth of 3365 metres. From the geological and geomorphological characteristics of the terrain, it can be concluded that the main cave canal continues further in the same direction, and it is possible that this cave is over 5 kilometres long.

The Miljacka II cave is a great challenge for speleological and speleodiving research. This is a highly obstructed speleological structure, with a maximum cave depth of 29 metres and maximum vertical difference of 50 metres. The cave has a typical large entrance chamber with divisions made of stone blocks, and it branches near the entrance (which in the past was used to keep sheep). The main channel, which is also the longest, extends in the direction NW-SE. In addition to the dry channels, the cave is also characterised by the presence of a temporary siphon, large lake and six (currently known) lake siphons. The third siphon is the first active siphon in the cave. During periods of high water, the first and second siphons also become active, and at that time, diving behind them is nearly impossible. The sixth siphon has been investigated and topographically mapped to a distance of 445 metres, though it continues further.

On the basis of the topographical sketches and collected speleomorphological data, based on its morphological type, the cave Miljacka II can be categorised as a layered and branching speleological structure. In terms of its genesis, the cave is classified as a tectonic-erosional (polygenetic) speleological structure, as both tectonic movements and the erosion and corrosion effects of water played the main roles in its formation. In terms of its hydrology, the Miljacka II structure has a constant water course, where the quantity of water depends on the general hydrological situation in the broader area. The cave is a valuable part of Croatia’s geoheritage. 

The entrance part of the Miljacka II cave is dominated by conglomerates and marly limestones, while the final channel is completely formed in marly limestones. The cave is periodically a strong source. During periods of high water, an underground river flows through the cave. The cave is dominated by erosion-corrosion rock relief with clastic sediments deposited from sporadically strong water flow or the tectonic splitting of blocks caved in from the channel ceiling. The channels sides and ceiling throughout the cave show strongly developed erosional forms, eddy pools and current flows, that indicate the strength of the water flowing through it. The cave is ornamented by calcite stalagmites and stalactites of exceptional beauty. Due to the frequent flooding, the stalagmites are caked in mud, so most are brown in colour. At the entrance to the cave, loose, black accumulations of guano (bat excrement) are visible, while the ceiling above these accumulations is covered by a shiny, black crust, which is likely formed from phosphate minerals created from the reaction of bat urine with the limestone.

The air temperature in the entrance part of the cave is under the influence of external weather conditions, and can significantly vary, with an average value of 13.9°C. Further into the cave, the climate is more stable, with an average value of 13.1°C.

To date, 54 taxa of invertebrates have been confirmed to inhabit the Miljacka II cave, of which 18 fall within higher systematic categories: mites (Acari), spiders (Araneae), centipedes (Chilopoda), beetles (Coleoptera), springtails (Collembola), millipedes (Diplopoda), true flies (Diptera), molluscs (Gastropoda), isopods (Isopoda), amphipods (Amphipoda), decapods (Decapoda), oligochaetes (Oligochaeta), harvestmen (Opiliones), bees, wasps and other flying insects (Hymenoptera), grasshoppers and crickets (Orthoptera), pseudoscorpions (Pseudoscorpiones), booklice (Psocoptera) and  bristletails (Diplura).

Among these taxa are numerous species that are endemic to the Dinarides mountain range and Croatia. For example, the cave centipede species Eupolybothrus cavernicolus is stenoendemic, and Krka National Park is the type locality. The cave is also inhabited by the olm, Proteus anguinus, an amphibian endemic to the Dinaric karst.

Research conducted in the Miljacka II cave to date has confirmed the presence of nine bat species, and the cave has been included on the list of Internationally important underground bat shelters (UNEP/EUROBATS).


Photo: Vedran Jalžić