Our directory of things of interest

University Directory

Dragons under the mountain: Exploring Slovenia’s caves

Slovenia’s Postojna cave, 30km southwest from the capital of Ljubljana Slovenia’s Postojna cave, 30km southwest from the capital of Ljubljana Postojna Jama
23 Feb
Laura Cole explores Slovenia’s caves to see its strange top predators and the new threats they face

‘Hel-lo’, I yell in the cave, giving in to childish impulse. But the noise does not bounce back as expected. Instead, for eight long seconds, the sound travels down the chamber before fading out in the darkness below. ‘It doesn’t reverberate because the cave goes on for so long,’ says guide Miran Mejak.

A group of us are standing at the highest point of ‘Big Mountain Hall’, well within the gut of Slovenia’s national treasure – Postojna Cave. The length of the place is the first surprise, we’ve travelled more than three kilometres by underground railway to get to this point, but the whole cave system is at least ten times that. The second surprise is its volume. We think of caves as small and cramped – a nightmare for claustrophobes –- but this part of Postojna Jama (Slovenian for ‘cave’) is a cathedral of space. ‘That’s why it’s called the Big Mountain,’ says Mejak. On its ceiling and sides, irregular alcoves appear and extend upwards into darkness. Twists, turns and folds warp perspectives like an MC Escher illusion come to life.

This dark world is surprisingly busy. ‘There are thought to be 150 different species that live in the cave,’ says Mejak. In fact, the cave is so rich in biodiversity, it has become ground zero for a specific branch of science: speleobiology – the study of cave animals. ‘The very first troglobite – an animal that never ventures outside – was found here, the slender-necked beetle,’ says Mejak. However, a larger and more famous troglobite also calls the cave home. A strange, translucent salamander called an olm, or as its known in Slovenia, a ‘baby dragon’. We are here to understand their world and how it is changing.

caveThe underground railroad connects some of the largest limestone caves (Image: Postojna Jama)


Postojna Cave is made of limestone, but its illogical shape is entirely shaped by the movement of water. Two million years ago, the Pivka river ran through the great mountain hall, creating the kilometres of chambers. It’s still going elsewhere in the system. ‘The river is always finding new levels,’ says Mejak. ‘It is now 40 metres below us tunnelling a new cave.’ As it moves further into the Earth it leaves behind a course of passages, like a snake shedding its skin. After the river disappeared from this level, a rockfall created the mountain of debris today’s visitors now stand on. After that, the stalagmites would have begun their slow ascent upwards, the stalactites downwards.

The cave formations alone are a sight to behold. Some look like spun sugar, others are folded into Vienetta-style crinkles. The food comparisons are shared among our whole group: there’s ‘streaky bacon’ for pink and white ripples of rock and ‘stiff peaks’ for a group of small stalagmites that look as though they’ve been recently whipped up with a fork. There are fine, spaghetti-like strands that cover whole ceilings and rooms with what look like walls of melted chocolate. As a whole, the cave has the look of being half-baked, a place still in the process of very slow metamorphosis. ‘The stalagmites grow at a pace of one millimetre every 40 years,’ says Mejak. Some of the columns here are as tall as buildings putting them at least a million years old.

‘Once the river has left a cavern, it is really the rainfall that makes everything,’ says Mejak. ‘The rain falling to the ground above us, encounters the soil for the first time, soil that is rich with CO2, creating a carbonic acid when mixed with water.’ Such acid would be weak enough to drink, but strong enough to slowly dissolve through the 90 metres of limestone above our heads. The water reaches the ceiling of the cave, then waits for a moment before dropping to the floor, leaving behind a speck of calcite. The calcite forms the stalactites on the cave’s ceiling and the stalagmites on the cave floor. An upper and lower jaw closing at the pace of aeons.

It is cold in the cave, but the colours are warm. ‘The natural colour of the limestone is clear, however, it reflects the colour of the minerals in the soil above us,’ says Mejak, ‘iron for the red and oranges.’ Occasionally there are pure white limestone formations that are translucent under the light. They are reminiscent of the strange see-through creatures that rule this underworld.

Olm 002Proteus Aguinus or a ‘baby dragon’ in the vivarium (Image: Postojna Jama)

‘They already know that you are here,’ says Primoz Gnezda, a speleobiologist and cave researcher. It’s unsettling. The animals, which are waiting in a group in the corner of a dark, glass tank behind him are almost completely blind. There are a dozen or so, all long, white bodies and tiny limbs, their snub noses raised in waiting. A group of proteus anguinus – olms – or as they are known in Slovenia, ‘baby dragons’.

‘They do have eyes, but they are hidden in their faces and are not very strong,’ says Gnezda. What they do have is photosensitive skin. They can feel light shining on them. ‘They also have special receptors in the head that can sense electromagnetic signals and – right now- our electronic devices,’ he says. He has built this proteus ‘vivarium’ that sits in one of the cave’s chambers, a tank that allows the public to view the olms away from the river. He describes how one day the tank was moved and its rocks were rearranged. With a fondness in his voice that he seems used to justifying, he says ‘as soon as we put them back in the water, they all gathered by the same rock again, even though it had been moved. The olms, they know the smell of home.’

Olm 016Proteus egg (Image: Postojna Jama)

The proteus has good reason to stay put. Its abilities have made it one of the most specially adapted animals in the world, and the top predator of this environment. ‘The cave is almost completely without food, but they manage to thrive here,’ says Gnezda. He should know. He has dived the sections of the underground caves inaccessible to the public and found hundreds of olms, occasionally swimming through lakes carpeted with their white forms. Unless disturbed, they barely move, so as to expend as little energy as possible. ‘When the river brings something to them, say shrimps, or tiny river crabs from the outside,’ he explains, ‘they take note, they wait, they ambush only when the time is right.’ The olm’s secret, it seems, is patience. They have been known to survive for as long as 12 years without food. As we watch the proteus group closely for ten minutes, one lifts a front paw and places it half a centimetre forward. It doesn’t move again.

The least explained characteristic of the proteus is its curiously long life. They can live to 100 (102 in the longest case), far outliving most salamander species’ lifespans of around 20 years. ‘All other salamander species are dead by the time the proteus even becomes sexually mature,’ Gnezda jokes. Even in adulthood, however, the baby dragon never quite grows up. It retains the red external gills of its larval form and the ability to regenerate limbs.

In spite of these advantages, the proteus is IUCN Red Listed as being ‘vulnerable to extinction’. ‘The truth is that they are completely dependent on the river and the caves,’ says Gnezda. ‘They cannot survive anywhere else.’ Experiments aren’t needed to prove this theory. Occasionally, strong storms and high water levels flush the olms out of the caves. However, the warmer temperatures of the outside mean they don’t survive for long. ‘To stay alive they need to stay in the caves around a clean and dependable water source.’ Worryingly, that dependable water source is beginning to change.

Postojna Cave adventure tour 5764 Valter Leban Postojnska jamaAccess to the lower, Pivka cave (Image: Postojna Jama)


To get a better look at the Pivka river, we need to descend to its current level. Approaching the cave system overland, a mile and a half dead north, the land seems to fold in on itself into a leafy crevasse with sheer sides. Finding the river at the bottom, we follow its banks until it disappears under into a wedge of dark. We follow it.

If Slovenia does have a fully grown dragon, the Pivka river is it. Lit by our headlamps, its possible to see the full might of the water foaming through the tunnel bends. ‘There are no stalagmites here yet,’ Mejak shouts over the gush. ‘The force of the water washes them all away.’ This movement of water through the caves brings a fresh supply of food and nutrients for the cave animals. However, it has begun to bring other substances too. ‘Industrialisation, farming and landfills mean the river now brings fertilisers and waste water into the cave system too,’ says Mejak. ‘Although they are underground, and seem isolated from the outside world, the caves are very porous to pollution from outside.’

pivkaThe Pivka river on a calm day (Image: Postojna Jama)

Its troglobites are sensitive to it too, and their numbers are known to be decreasing. ‘It is impossible to know how many are in there,’ says Gnezda, ‘so we don’t know how near or far they are from extinction. What we do know is that they are vulnerable to it.’

This partly explains the vivarium: Postojna uses its favourite cave animal to campaign for better waste and water management in the surrounding countryside. ‘If we can show people how fascinating the proteus is, and how much we still have to learn about it, hopefully we can persuade them to protect the caves from pollution, for the benefit of all of the species.’

Crucially, one of Gnezda’s recent discoveries made international news. ‘We found eggs in the aquarium,’ he says. ‘It was the first time we have managed to raise captive olms here in Postojna.’ In 2016, the eggs hatched into 22 juvenile olms – ‘baby’ baby dragons, if you like. ‘It’s captured a lot of attention and gives us the chance to study young ones,’ he explains. ‘But on a more serious note, they are a fallback should anything happen to the wild population.’

2018 marks the 200th anniversary of the discovery of Postojna's inner cave system. The Big Mountain Hall and Baby Dragon Vivarium can be seen on the Standard tour. The Pivka Cave can be accessed on an Adventure Tour.

Slovenia travel information can be found at www.Slovenia.info

geo line break v3

Free eBooks - Geographical Newsletter

Get the best of Geographical delivered straight to your inbox by signing up to our weekly newsletter and get a free collection of eBooks!

geo line break v3

Related items

NEVER MISS A STORY - Follow Geographical on Social

Want to stay up to date with breaking Geographical stories? Join the thousands following us on Twitter, Facebook and Instagram and stay informed about the world.