Slovenia has a plethora of karst caves but Križna Cave is one of the few that can be visited on a boat. The number of visitors is restricted and those wishing to see more than one lake should book well in advance.

Situated near the village of Bloška Polica in south-central Slovenia, not far from the intermittent Lake Cerknica, Križna Cave is the only naturally preserved tourist cave in the country, which means there is no electric lighting or paved pathways.

The cave is known for its underground lakes and sinter dams, formed by deposits of limestone in the river that runs through the cave system before disappearing and flowing out into Lake Cerknica. The river has formed 45 such lakes, 22 of which can be visited on a rubber raft, but only on the longest tour.

Since the elimination of carbonate from the water at rapids is more than ten times faster compared to lake water, tufa barriers form there, creating lakes up to seven meters in depth, the cave's website explains.

To preserve the cave and its fragile natural habitat as much as possible, the association managing the cave has restricted visitor numbers considerably.

Patience rewarded by more treasures

Visitors are given rubber boots and flashlights. Those opting for the shortest tour, taking just over an hour, will see roughly one fifth of the cave. Guides will take them through the dry part of the cave and on a short boat ride on the first of the series of lakes.

There are not many stalagmites and stalactites in this first part of the cave, but the cave is much richer in various limestone formations further on, says Gašper Modic, the head of the association managing the cave.

In July and August four to five such short tours are available daily, for a group of up to 20 people. Most other times visits are limited to one or two daily.

Only 1,000 visitors a year can take a four-hour tour, which takes in 13 lakes. To see the furthest parts of the cave, it is possible to book a seven-hour tour, but it is available for only up to 100 visitors a year.

"The main reason for the restrictions are the sinter barriers created by the river because we don't want visitors to ruin them while crossing them on foot," Modic has explained for the Slovenian Press Agency.

Four-hour tours are booked up three to four months ahead for weekday visits and about half a year for weekend tours. The seven-hour trip needs to be booked at least a year and a half to two years ahead. Modic notes that the wait times have been made longer by the Covid-19 pandemic.

Cave bear bones

The cave is also known for cave-bear remains. Due to the large amount of the bones it is deemed possible that the cave had been home to the bears for tens of thousands of years. That is also shown by many bear marks on the rocky walls.

The bears that inhabited the cave became extinct around 25,000 years ago. These were very large animals, as can be seen from the skull of one of them, which is on display in the cave under a glass case.

"The largest bear skull in the world, 56 centimetres long, was found in Križna Cave. The bear is thought to have weighed 1,500 kilogrammes, despite being a vegetarian," Modic says.

The first known visitors to the cave were people from the Metal Age. Archaeologists have found many ancient ceramics in the entry area, the oldest of which are almost 5,000 years old. People never lived in the cave but used it mainly as a hideout.

The oldest signatures on the cave walls prove that people have been visiting the cave since as far back as 1557. In the following centuries, the locals visited Križna Cave, but only rarely. Mostly they visited the entry part of the cave and most of them would not even venture as far as the first lake.

The cave had not been explored deeper until a Ljubljana secondary school teacher led his students on an excursion there in 1926. Tours for visitors have been available since 1952 and since 1998 they have been organised by the Association of the Križna Cave Fans.

Their president says the cave is very safe to visit. During heavy rainfall, the water level can rise considerably, and in 2014 it rose by as much as 15 metres, but it is rising slowly, with a one-day lag.

For further details and bookings, click here.