Most fameous pits of the Northern Velebit National Park

In the area of Northern Velebit, especially in the strict reserve Hajdučki and Rožanski kukovi, a large number of speleological structures make the area of the Park a special locality. By the end of 2020, more than 600 speleological structures have been discovered, of which pits predominate. The density of speleological objects, long verticals, underground siphons and lakes and endemic cave fauna have brought this area to the very top of the world karst morphology. According to their morphological and microclimatic characteristics, some of them are among the most important speleological structures in the world.

 

1. LUKINA JAMA – TROJAMA

The Lukina jama – Trojama system is the most vertical pit in the world with a depth of 1431 m. It is located on Hajdučki kukovi, with entrance to Lukina jama sits at 1438 m above sea level and the entrance to Trojama is at 1475 m above sea level.

The Lukina jama – Trojama system is tectonically predisposed, and was created by water corrosion and erosion. Most of the pit follows a meandering pattern, which indicates that it used to contain more water in the past. As its depth increased, the share of erosion in the overall weathering process increased as well, creating a number of erosion features, in particular dolly tubs. There are horizontal syphon passages that have developed at the very floor.

The presence of ice was discovered during the exploration of the Lukina jama – Trojama pit system in 2011, up to the depth of 560 m below the level of the present entrances, which makes it the subterranean structure with the deepest ice level on record in the world. To determine the upper age of the ice, the dating of wood residue discovered in the ice at the depths of 120 m and 160 m in 2011 was performed, and analyses showed that their upper age was 410 ± 75 years.

One-year microclimate research revealed a remarkable stability of microclimate conditions at greater depths. In contrast, substantial changes in the amounts of ice deposits were observed at shallower depths in the pits of Northern Velebit in the past 20 years, which serves as another proof of recent climate change.

Stalactites at the considerable depth of 945 m in Lukina jama, which were found to be 50,300 ± 1,100 years old, were another important discovery.

Lukina jama – Trojama is also known for deep syphon lake exploration. Zoran Stipetić “Patak” and Teo Barišić were the first to attempt a dive in 1994, and to cross a length of 157 m in the syphon. Exploration continued in 2010, when cave divers Ivica Ćukušić and Robert Erhardt dove to a length of 135 m and a depth of 21 m. As the expedition continued, Branko Jalžić dove even deeper into the submerged passage, to the depth of 40 m. Vedran Jalžić and Petra Kovač Konrad made the descent and the dive across the syphon in 2013. They descended 20 m deeper, increasing the known depth of Lukina jama to 1431 m. The two speleologists drew a 1:500 detailed topographic map, recorded videos, and collected fauna and sediment samples for analysis in one of the world’s most challenging expeditions at the time.

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2. NEDAM

The Nedam pit, discovered in 1997, is located in the southeast section of Hajdučki kukovi, west of Golubić, at 1430 m above sea level. In 2020, speleologists managed to penetrate to a depth of 1,226 m, but in 2022 new underground channels were explored to a depth of 1335 meters, which makes it now the second deepest pit in Croatia.

Nedam is a complex pit, composed of a number of short verticals and inclined meanders. Its distinctive, suddenly narrowing passages make it technically challenging to explore.

In addition to Nedam being tectonically predisposed, the mechanic force of water played the most important role in its formation.

The pit was named Nedam after an anecdote. Since it is made up of very narrow, almost impassable passages, a speleologist got stuck in a narrowing meander at the depth of 315 m and was unable to come out unassisted. It took his colleagues who happened to be there full three hours to extract him. They then decided to name the pit Nedam (loosely translates as “I won’t let you”) because it would not let them get any further. However, in the following expeditions, investing a considerable effort and perseverance, the speleologists were nevertheless able to get past the squeeze at the 315^th metre and enter a new inclined vertical, 247 m long, from which they were able to proceed to the floor of the pit.

When they came close to the pit’s lowest point, the speleologists encountered a syphon lake. The syphon was explored during an expedition in 2020, when Jenny Barnjak dove into it, to the depth of 6 m and the length of 35 m. Behind this segment of the syphon, she discovered it continued in an air pocket. The dive into this syphon was another landmark event in cave diving and deep pit exploration.

During the research in the pit in 2022, a series of extremely demanding caving dives were performed in the siphons in the lower parts of the pit. The researchers re-dived one siphon that had been diving for the past two years and found two more siphons that had not been dived. During the 2022 expedition, more than new 300 meters of length and 85 meters of depth were topographically recorded and drawn.

Many types of fauna were found in the pit. The discovery of Meštrov’s leech in shallower segments of the pit was of particular interest, since this species was only found at much greater depths in the other deep pits. The pit has strong air circulation and is decorated by cave corals on passage walls.

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3. SLOVAČKA JAMA

Discovered in 1995, Slovačka jama is located on Mali kuk, at the height of 1520 m above sea level. It has been explored to a depth of 1324 m and is the third deepest pit in Croatia.

Its morphology is complex, with extremely branched passages, frequent exchanges of horizontal and vertical sections, and knee-shaped and partially multilevel morphology. Horizontal passages, some of which are hydrologically active, form the deepest segments of the pit.

The analysis of stalactite samples collected from the cave showed them to be 37,000 years old. A large Croatobranchus mestrovi leech population, along with a number of other animal species, was discovered in the pit.

Too narrow passages and the syphon lake on the floor of the pit prevented more detailed exploration of its depths for years. However, using proper gear, speleologists managed to traverse the narrow passages leading to the lake syphon at the depth of 1320 m in 2019. Cave diver Branko Jalžić was able to dive into the syphon passage to the depth of 4 m and the length of 10 m.

The narrowness of the passage prevented him from advancing any further, since the equipment could not get through. The possibility of getting through in the future exists, but only using gear that is adapted to the morphology of the sunken passage. With this cave dive, Slovačka jama became the world’s 28^th deepest pit.

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4. VELEBITA

The Velebita – Dva javora system, discovered in 2003, is located on Rožanski kukovi, near Crikvena. Velebita is at 1550 m, and Dva javora at 1557 m above sea level. It has so far been explored to the depth of 1026 m and is the fourth deepest pit in Croatia. 

This knee-shaped pit has a complex morphology up to the depth of 228 m, where a passage links it with the Dva javora pit, whose entrance is located very close to the entrance into the Velebita. The system composed of these two pits was named Velebita – Dva javora. The entrance into a long, 513 m shaft, named Vertikala Divke Gromovnice, is located at the juncture of Velebita and Dva javora. Vertikala Divke Gromovnice is the world’s longest interior shaft. From the 105^th metre, and for a further hundred metres, this shaft, elliptical in diameter, for the most part remains 8 x 3 m in size. After the 210^th metre, it expands to 40 x 15 m, and retains this diameter all the way to the floor. The pit ends in an enormous cavity, which was named Podzemljarova dvorana. It is located at the depth of 1026 m, and its dimensions are 70 x 60 m.

A half-hour film was made about the expedition into the pit, biological samples were collected, and physical and chemical measurements were conducted.

A number of troglobiontic organisms were found in the collected samples, notably tiny snails from the Zospeum genus, a number of terrestrial woodlices, millipedes from the Haasia genus, pseudoscorpions from the Neobisium genus, and a number of springtails and beetles from the  Astagobius and Speleodromus genera. The largest colony of endemic cave leeches belonging to the species Croatobranchus mestrovi (Meštorov’s leech) and a single specimen of troglobiontic harvestmen, probably from the genus Hadzinia, may be the most important findings from the Velebita pit.

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5. MEDUZA

Discovered in 1999, Meduza is located on Rožanski kukovi, at the height of 1600 m above sea level. Its entrance is barely noticeable. The pit is 706 m deep and is located on the floor of a sinkhole, where crevices converge. Meduza is famous for its interior shaft that is 450 m long, which makes it the world’s second longest interior shaft. Vertikala Divke Gromovnice in the Velebita pit is the longest.

Morphologically speaking, Meduza is a knee-shaped pit. It has several shafts, with meanders branching out into less worn rocks. The meanders are not very wide, and are often very narrow and of an elliptical shape, exhibiting traces of fluvial erosion. Dolly tubs formed at their entrances due to sudden changes in water energy, caused by narrowing passages and changing directions of the watercourses, as well as the formation of vortexes and the consequent destructive effects of the water and the stone material in the watercourse. In some meanders, the watercourses cut downwards, reshaping the meanders into canyon-type passages.

The largest shaft in the pit, called Bojim bojim, is 450 m long and descends to a depth of 679 m. In parallel with this shaft, across a buried passage at the depth of 570 m, there is an entrance into a parallel shaft, measuring 136 m in length, which reaches the deepest point of Meduza known to date, at the depth of 706 m. The shaft ends in a large cavity measuring 20 x 50 m. 

Both geological and biological research has been done in the pit. Fauna samples were collected and identified, most of them troglobiontic and completely adjusted to subterranean conditions, with no pigment or sense of sight. A genus of Zospeum snails was identified, along with genus Alpionicsus, the woodlice crustaceans, dipterans, harvestmen, and others.

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6. PATKOV GUŠT

The entrance into the Patkov gušt pit is located in a sinkhole on Gornji kuk, at the height of 1450 m above sea level. The pit was discovered in 1997 and has been explored to the depth of 553 m. The distinctive feature of Patkov gušt is a continuous shaft, extending from the entrance to the floor, which makes it the pit with the world’s second longest shaft. It was named after Zoran Stipetić Patak, speleologist and diver in Lukina jama pit.

Its morphology is simple, but its walls and the deposits of snow and ice in the pit make its exploration extremely difficult.

Speleologists made their way down into the pit by passing through narrow crevices between the ice and snow, up to the depth of 105 m, where they encountered the narrowest passage, 2 x 1.5 m in size. Ice and snow melting causes considerable water dripping, which intensifies the deeper you go. Up to the depth of 300 m, the walls are fully or partially covered with an ice cover, and ice blocks melting and breaking away are a constant threat.

The pit measures ends in a large cavity measuring 40 x 30 m. The pit’s lowest point is located at the northern end of the cavity, which features a shallow passage where the water drains through the mud and rocks deeper into the mountain.

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