Planinska jama
Planiska jama (Planina Cave) is one of the more extensive underground systems of the Slovenian karst, where geological history, hydrological processes and long-standing human curiosity intersect. With 6,656 metres of surveyed passages, it is among the longer Slovenian water caves, yet its significance goes beyond its length. It offers a clear view of classical karst processes, from the dissolution of limestone and the formation of passages to the development of underground watercourses and specialised ecosystems.
The entrance to the cave lies in a blind valley on the southwestern edge of Planinsko polje, beneath a vertical rock face approximately 65 metres high. The formation of this valley is linked to the long-term action of underground water, which widened the cave passages and gradually caused the collapse of the surface above. The initial section of the cave is spacious, allowing a gradual transition into the darker and colder interior, where constant humidity and characteristic silence prevail, interrupted only by dripping water and the sound of underground streams. Water is the key factor in the formation and ongoing development of Planinska jama.
About 500 metres from the entrance lies the confluence of the Pivka and Rak rivers, one of the most important hydrological features of the European karst. The Pivka flows in from Postojnsko polje through Postojnska jama, while the Rak arrives from Rakov Škocjan. Their meeting in a large underground chamber is a unique phenomenon, as it represents the largest known underground confluence in Europe. The combined flow continues as the Unica River, which soon reappears on the surface at Planinsko polje. This transition from the underground to the surface is an important part of the hydrological system of the Inner Carniola karst.
The cave is formed in thick layers of Cretaceous limestone, which is highly susceptible to chemical dissolution. Water enriched with carbon dioxide has, over long periods, widened cracks and shaped passages and chambers. Planina Cave contains both active water passages and fossil passages that indicate former, higher water levels. Speleothems occur mainly in areas where water does not flow continuously, while in active water passages they are less pronounced due to erosion.
The cave hosts several specialised subterranean organisms. Among them is the olm (Proteus anguinus), characteristic of the Dinaric karst’s aquatic systems. Also present are cave-dwelling crustaceans, snails and numerous small invertebrates adapted to life without light. Planina Cave is an important winter roost for bats, which rely on its stable conditions. Due to its biodiversity, the cave is included in the Natura 2000 network.
The cave was known in the Middle Ages, but systematic exploration began in the 19th century. Researchers faced fluctuating water levels and difficult-to-access passages, yet gradually revealed the cave’s structure and confirmed hydrological connections between Postonjska jama, Rakov Škocjan and Planinsko polje. In the 20th century, cave divers explored the siphons and expanded knowledge of the system. Despite extensive research, the cave still offers opportunities for new discoveries, particularly in flooded sections.
Planinska jama is an important part of a wider karst system that connects several poljes and watercourses. It contains key elements of classical karst, such as underground rivers, siphons, collapse features, karst springs and ponors. Because of the interconnectedness of underground waters, the area is sensitive to environmental changes and pollution, making the protection of the cave and its catchment essential for preserving natural processes.
Planinska jama is therefore a significant natural feature that combines geological, hydrological, biological and historical characteristics of the Slovenian karst. Its structure, watercourses and the underground confluence of the Pivka and Rak rivers provide insight into the functioning of karst processes that shape the landscape. Due to its position and connections with other karst systems, it is important for understanding the broader hydrological network of Inner Carniola. Preserving the cave and its surroundings is essential for further research and for protecting the natural heritage characteristic of this region.
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virtual excursion (2009)