Our guide turns out the lights and suddenly there is nothing. Just total darkness, the sound of gentle dripping and a creeping feeling of unease. The switch is flicked back on and the shadowy world that lies deep beneath the Karst returns. I’m in Vilenica, thought to be the first cave in the world ever opened to tourists, with records of visitors dating back to 1633. It’s a magical sight: a grand antechamber sculpted through erosion, filled with soaring stalagmites and plunging stalactites streaked in shades of red, terracotta and orange by iron oxide, and dotted with shimmering crystals. Vilenica is just one of a network of thousands of caves located in the Karst region of western Slovenia and eastern Italy, which is known for its porous, soluble limestone rock. Above ground, this creates a distinctive landscape, filled with rocks bearing lined striations and pockmarked by hollows known as dolines, where the limestone has collapsed underneath. But below ground is where it’s really special, with enormous caves, sinkholes and subterranean rivers. Later in the day, …