Slovenia: Skocjan Caves
The entire south of Slovenia is one massive karst region with some of the world’s largest and most spectacular caves. The Skocjan Caves are a UNESCO World Heritage site for its massive underground canyon and river (imagine Grand Canyon with the Colorado river – all underground). It’s just enormous and a track winds it’s way along precipitous cliffs with a bridge crossing, 300 feet above the river. And this is all underground!