Postojna Cave – Slovenia (15/20)

A marvel of nature, Postojna Cave in Slovenia stretches 24 kilometers (15 miles) and is one of Europe’s most famous show caves. Visitors travel through its immense chambers and winding passages on a train ride before exploring on foot. The cave system features delicate formations of stalagmites and stalactites, and it is home to the rare and enigmatic olm, or “human fish,” a species of aquatic salamander.