One of the most monumental underground features in the world, Slovenia's Skocjan Caves seem to have sprung directly from the mind of a fantasy author.
Often referred to as the "Underground Grand Canyon," the Eastern European cave system features one of the largest subterranean chambers in the world. The biggest room in the limestone cave network, known as "Martel's Chamber," is a gaping canyon which was formed by the underground flow of the Reka river, which still rushes along the bottom of the chasm.
Thanks to the amount of water that flows through the canyon, the caves also hold a number of eye-catching geological features such as limestone pools, travertine terraces, and giant stalagmite formations. The cave's unique spaciousness also hosts a number of species of bat, countless unique bio-organisms, and even a variety of fish that have never seen the light of day. A bridge has been built spanning the chasm so visitors can experience the colossal geologic divide.
Archeological research in the area found that the caves were home to humans as far back as 10,000 years in the past. It's likely that the caves were used by ancient people for shelter, and, although there is absolutely no science to support it, the Skocjan Caves also seem like they may have been the perfect lair for a dragon.