Škocjan Caves, Slovenia. The one of the top 10 attractions in Eastern Europe. They’re also on the UNESCO World Heritage. They live up to their reputation by being one of the largest underground canyons in the world with the Reka river still carving through it. At 60 meters wide and 140 meters deep, this canyon is a fraction of the Grand Canyon’s size, but the fact that it’s all underground makes it feel bigger.