Board an underground train and plunge into Europe's largest show cave — a cathedral-scale world of towering stalagmites, glittering cave pearls, and the five-metre white stalagmite known as the Brilliant. The 90-minute guided tour covers the highlights of a 24 km karst system that took two million years to form, including a stop to spot the cave olm — Slovenia's legendary "baby dragon," a pale amphibian that lives its entire life underground.

Wedged into a 123-metre cliff face, Predjama is the world's largest cave castle and one of Europe's most startling sights — a medieval fortress that looks like the rock simply grew around it. Inside, the story gets better: 15th-century knight Erazem defied the Holy Roman Emperor's siege for over a year, smuggling in supplies through a secret tunnel carved into the cave system behind the castle. How it all ended is a tale best heard on the spot. Wander the chambers, find the hidden passages, and let the legend unfold around you.