The Škocjan Caves (Škocjanske jame) are a limestone cave system on the Karst Plateau of south-western Slovenia, near Divača, where the Reka River drops underground and flows through a vast underground canyon. The explored length of the caves is about 6.2 km (3.9 mi); the main gallery, Hanke’s Channel, is about 3.5 km long and more than 140 m (460 ft) high, and Martel’s Chamber, with a volume of 2.2 million m³, is considered the largest underground chamber yet found in Europe. The caves were added to UNESCO’s World Heritage List in 1986 and designated a Ramsar wetland of international importance on 18 May 1999. After leaving the caves the Reka travels roughly 34 km beneath the plateau before resurfacing at the Timavo springs in Italy, a short distance from the Adriatic. This guide follows the water from its source to the sea.
Snežnik and the Brkini Hills: Where the Reka Gathers Its Water
The river that made the caves does not start on limestone at all. The Reka rises in Croatia as Vela voda, “Big Creek”, on the southern side of the Snežnik plateau, and crosses into Slovenia where it is first known by the Slovene equivalent, Velika voda. Its whole course is 54 km (34 mi), of which 51 km lies in Slovenia. Snežnik itself is a broad karst plateau of about 85 km², topped by Big Mount Snežnik at 1,796 m (5,892 ft), the highest non-Alpine summit in Slovenia. Because it stands only 28 km from the Adriatic, it catches up to 3,000 mm of precipitation a year, and it is snow-covered in winter.
From there the Reka runs north-west down a valley past Ilirska Bistrica, a town that grew rich on sawmills and timber sent to the ports of Trieste and Rijeka, and then along the northern edge of the Brkini Hills. The Brkini are a strip of country about 25 km long and 7 km wide, covering 180 km², with a central ridge at 700 to 800 m, and they are built mainly of flysch, a layered mix of sandstone and marl. Flysch is impermeable, so rain runs off it into surface streams instead of disappearing into cracks. That is the key to Škocjan: the Reka gathers most of its water on this non-karst ground, arrives at the limestone already a full river, and has the volume to cut a canyon when it meets rock it can dissolve. The river has a pluvial regime, so its flow follows rainfall rather than snowmelt. The whole Reka watershed, some 450 km², is treated as the park’s area of influence.
Škocjan Village, Its Natural Bridge and the Two Collapse Dolines
The plain where the river meets the karst, the Škocjan Karst, lies between 420 and 450 m above sea level. It looks flat, but it is pocked with sinkholes, and right at the contact the Reka plunges into a gorge. The village of Škocjan, which gives the caves their name, sits on a natural rock bridge above the river. Beneath it the Reka has carved Mahorčič Cave and Marinič Cave, and on either side the roofs of former caverns have fallen in to form two great collapse dolines, Mala dolina (Little Collapse Doline) and Velika dolina (Big Collapse Doline). The village is named after its church of St Cantianius (sv. Kancijan), and its medieval ground plan has survived.
Škocjan Caves Regional Park, set up by Slovenia after independence in 1991, protects just 4.15 km² around the caves, the Reka gorge and the dolines, including the villages of Škocjan, Betanja and Matavun. In 1996 the two dolines, the Okroglica abyss and the walls of the Reka valley within 150 m of Mahorčič Cave were declared natural monuments. People have been drawn to this spot for more than ten thousand years. Finds in Fly Cave (Mušja jama) show contacts with the Greek world, and a cave sanctuary was used here from the Bronze Age into the Iron Age, about three thousand years ago.
Hanke’s Channel and Martel’s Chamber: Inside the Underground Canyon
At the bottom of Velika dolina the Reka disappears into the rock. The caves have formed in a layer of Cretaceous and Paleocene limestone about 300 m (980 ft) thick. Inside, the river runs through a canyon, then turns north-west before the Cerkvenik Bridge and continues along Hanke’s Channel (Hankejev kanal). This passage is about 3.5 km long, 10 to 60 m wide and more than 140 m high, and it widens at intervals into chambers. The largest, Martel’s Chamber, holds 2.2 million m³ (78 million cubic feet) of space.
Mapping the canyon took more than a century. The first written mention of the caves is attributed to the Greek writer Posidonius of Apamea in Antiquity, and they appear on the Lazius-Ortelius map of 1561 and in Mercator’s atlas of 1637. In 1689 the Carniolan scholar Johann Weikhard von Valvasor described the sinking of the Reka and its underground course. Systematic exploration began with a speleological expedition in 1884; explorers reached the shore of Dead Lake (Mrtvo jezero) in 1890. In 1904 local men climbed the 60 m wall of Müller Hall and found Silent Cave (Tiha jama), and in 1990 Slovenian divers swam through the Ledeni dihnik siphon and added more than 200 m of new passage. The river leaves the explored cave through a relatively small siphon at the far end of the canyon, and that narrow exit shapes everything that happens inside when it rains.
When the Reka Floods: Water Rising 132 Metres Underground
In a high-water period the Reka can carry more than 300 m³ of water per second, far more than the end siphon can pass. The water backs up inside the canyon. After heavy rain it usually rises by up to 30 m, and the highest rise ever recorded was 132 m, which would fill almost the full height of Hanke’s Channel. Floods have reshaped the visitor routes: the first tourist section, through Marinič Cave, Mahorčič Cave and Mala dolina, was open by 1933 but was badly damaged by a flood in 1963, and the area was renovated in 2011 with a new steel bridge.
This rise and fall is why the system counts as a wetland as well as a cave. Together with the underground Reka, the caves form one of the longest karst underground wetlands in Europe, the basis of their Ramsar listing. The collapse dolines, with their own cool, damp microclimate, shelter rare birds and several bat species, while the cave water and passages hold specialised cave animals, both aquatic and land-dwelling. Visitor numbers run at roughly 100,000 a year, and tours are given in Slovenian, English, Italian and German.
Under the Karst Plateau: The Reka’s Hidden 34 Kilometres
Beyond the last siphon the river vanishes. Most of its course under the Karst Plateau is unknown and can be reached only in a few deep shafts, up to about 320 m deep, where siphons stop divers going further. Sources disagree on how far it travels: about 34 km in the caves’ own description, about 35 km by the park’s figure, and 38 km in accounts of the river itself. The spread shows how loosely the route is known.
Above it lies the Kras, the plateau that gave the world the word karst. It rises steeply from the surrounding land and averages 334 m above sea level. The Slovene part covers 429 km², holds about 19,000 people in exactly 100 settlements, and is centred on Sežana, with Divača, Dutovlje and Komen as rural centres. Forest now covers only about a third of it after centuries of clearing. The plateau is riddled with caves: Vilenica, Lipica, Divača Cave (89 m deep) and Kačna jama, or Snake Cave (280 m deep), all lie near Škocjan. Much of the early exploration was driven by Trieste’s need for drinking water. Speleologists searched the plateau’s shafts for the underground Reka, and Grotta Gigante on the Italian side was first explored in 1840 by Antonio Federico Lindner as part of that search for Timavo water for the city’s aqueduct. You can read more about the plateau’s surface landforms in our guide to the Karst Plateau.
Tracer studies have shown that Reka water does not emerge in one place. It feeds springs along the Adriatic coast between Trieste and Monfalcone, mixing on the way with rain that soaks straight into the limestone.
The Timavo Springs at San Giovanni di Duino
The river comes back to daylight in Italy as the Timavo (Slovene Timava), which rises from four sources near San Giovanni (Štivan) close to Duino in the Province of Trieste. It is a stream only about 2 km long, flowing into the Gulf of Panzano, part of the Gulf of Trieste, some 3 km south-east of Monfalcone. Modelling suggests that about one third of its flow comes from the Reka and two thirds from rain that infiltrates the Karst Plateau, with smaller contributions from other sinking rivers, including the Vipava, Soča and Raša.
The Romans knew the place well. Livy, Strabo and Virgil all mention the river; Virgil wrote of nine streams bursting from a mountain, and a Roman settlement near the springs was called Fons Timavi. From the Snežnik slopes in Croatia to the gulf below Duino, the journey links three countries and two rock types, impermeable flysch and soluble limestone, and for some 34 km of it the river runs where no one can follow. The same pattern of sinking rivers, collapse dolines and resurgent springs repeats across the Dinaric region, and our explainer on karst landscapes around the world shows where else limestone has swallowed a river whole.