The Skocjan Caves

In Slovenia:

 

Often referred to as the “Underground Grand Canyon,” the Eastern European cave system features one of the largest subterranean chambers in the world. The largest 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 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 has never seen the light of day. A bridge has also been built, spanning the chasm so that visitors can experience the colossal geologic divide.