A single chemical equation, the equilibrium reaction of solution and precipitation of limestone, is responsible for a variety of fascinating phenomena commonly referred to as karst: Caves, sinkholes, percolating rivers, karren, polje and so on (cf. my book The Formation of Mountains). The term karst, like many other terms of geomorphology, derives from the South Slavic languages. The karst of the Dinarides stretches from Slovenia to Montenegro and in this region some spectacular karst phenomena can be seen.
In Slovenia, two caves in particular are famous and both are worth a visit, despite the high entrance fees. In Postojna, the crowds are a little off-putting, but the forests of stalactites are truly impressive and make many other caves look pale. A good part of the route is covered by an underground train, which makes it feel a bit like riding through cheap scenery in a theme park. But the stalactites are real and stand close together. Then follows a walk through the most beautiful halls. A few kilometres away there is an interesting castle built into the mouth of a large grotto, it is worth at least a short photo stop.
The Škocjan caves are almost more fascinating than Postojna, but so different that a comparison is hardly possible. The first part of the tour is still comparable to many other caves, but then you approach the river Reka, which thunders through a deep underground gorge. The illuminated path that leads through this abyss seems tiny in the dimensions of the cave and mist wafts up from below. A truly magical place. Finally, you emerge into daylight inside a huge doline (sinkhole) and take a lift back up. We also do the second (new) tour, which leads from the other side through the “underground gorge” to the doline. This part is more like a tunnel, but is also fascinating. Under no circumstances should you skip the vantage point with a view over the huge doline (to be more precise, there are two dolines separated only by a narrow ridge with a rock arch through which the river rushes).
If you want to see karst phenomena without the crowds of tourists, you should take a walk (approx. 2-3 h) along the nature trail through Rakov Škocjan (which is not near Škocjan cave, but between Postojna and Cernica: signposted from the Unec motorway exit). The hikers’ car park is at the small rock gate Mali Naravni Most. Here a river comes out of a cave and flows through a system of sinkholes, tunnels and rock gates. It is definitely worth descending the steep path and exploring the sinkholes from below as well, before following the river on the nature trail at the top. On the way, you will encounter other less spectacular karst phenomena before the river flows through the large Veliki Naravni Most rock gate and disappears into a cave again after a short gorge.
Not far from here, Cernica lies in a polje. Polje are the name given to the wide plains common in the Karst region, which are surrounded all around by steep slopes and have no overground drainage. All the water disappears underground through ponors and the dissolution of the limestone on the slopes leads to an enlargement of the plain. In Cernica polje there is a seasonal lake whose water level depends on the groundwater level and the water in caves and crevices.