Skocjan Caves, Slovenia

One of the most important cave systems in the world, Slovenia's Skocjan Cave system was added to UNESCO’s list of World Heritage Sites in 1986. Entering the caves at the foot of a steep gorge below the village of Skocjan, the Reka River runs underground for 21 miles (34 kilometers) through the massive underground canyons, gorges and caverns, surfacing at Monfalcone to join the Timavo River. This underground channel is over 460 feet (140 meters) high, expanding at points into enormous underground chambers, including Martel’s Chamber, the largest discovered underground cavern in Europe.