It’s often said that the Age of Exploration is over. Mount Everest has been scaled, Antarctica traversed, every inch of every vast desert or rainforest mapped with satellite imagery and laser ranging technology. Modern-day explorers must look to other frontiers not yet mapped in any atlas:

The bottom of the sea, for example, or faraway Mars. But sometimes we forget another unmapped mystery, often nearer at hand: subterranean caves.

The world’s largest cave is on the border of Vietnam and Laos.

Half a million tourists visit Mammoth Cave in Kentucky every year.

Mammoth Cave is the world’s longest cave system, with over 405 miles of passageways known and more being discovered every year.

But it doesn’t contain the world’s single biggest cave. For that, you’d have to trek through the dense jungle of Vietnam’s Phong Nha-Ke Bang National Park, home of Son Doong Cave.