The illuminated, 20km-long karst cave at Postojna

Millions of years in the making, Slovenia's limestone underworld is one of the country's top attractions.

A subterranean network of caves lurks beneath the landlocked terrain southwest of the Ljubljana.

The most visited of the caverns is Postojna Cave, where the Pivka River has eroded the limestone rock to create a 21-kilometre-long network - one of the longest caves in the world. In the depths of this illuminated natural temple, the river flows along the cave floor, while above, stalagmites and stalactites go about their business at an evolutionary pace, elongating at a rate of one millimetre every 30 years. Postojna Cave maintains a temperature of 8°C, so be sure to wrap up on the guided tours, which point out oddities such as centuries-old cave graffiti and includes a short electric train ride through the tunnel.

While the town of Postojna itself hasn't a whole lot going for it, the surrounding area is positively riddled with caves, many of them without lighting or walkways and only fit for exploration by experienced cavers.

Lying some distance southeast of Postojna, Krizna Cave is a real hidden gem with 22 underground lakes and harbouring the fossilized remains of cave bears. Here, you can float through the cave complex on a rubber raft.

Backtracking to the Postojna Caves and heading 9km west, you'll come to Predjama Castle, a 700-year-old, four-storey fortress built into a mountaintop cave. Beneath the castle is - you guessed it - another cave. You'll need a torch to shine a light on its six kilometres of mysteries.

Some 33km southwest of Postojna is the less-visited but perhaps most dramatic karst cavern of them all - the Skocjan Caves. This UNESCO World Heritage is as close as it gets to Jules Verne or Indiana Jones fantasies, with underground chambers crossed by natural bridges and complete with lakes, subterranean waterfalls and rapids.