These are the impressive Škocjan Caves in Slovenia , where the Reka River disappears deep underground. People have lived in the caves since prehistoric times, and the internal atmosphere even has its own eco-system.
The ultimate guide to exploring Slovenia’s Postojna Cave and Predjama Castle
Postojna Limestone Cave - Slovenia * The limestone cave is Europe's most expansive, measuring 487 feet long and 377 feet deep. Guided tours through the wonderland of stalactities and stalagmites are conducted by miniature electric trains.