Thousands of bats stream out of caves every night on the edge on Thailand's second largest park.

A limestone wonderland of water and subterranean secrets.

Ancient paintings known as the "Sistine Chapel of Cave Art."

Mother Nature outdoes herself with this stunningly beautiful set of caves carved into marble.

The longest continuous lava tube in the continental U.S. offers 2.5 miles of dark explorations for the adventurous.

Cretean quarry suggests real-life site for ancient Greek myth of the man-eating Minotaur in his labyrinth.

A miniature train-ride through Slovenia's largest cave system.

This astonishingly geometric cave has inspired everyone from Jules Verne to Pink Floyd.

Enchanting cave with a river crashing through the roof.

The world's longest known cave system.

Cave on extinct volcano, once home to famous mathematician Pythagoras.

Rock-cut cave complex filled with ancient Buddhist artwork.

This complex of vertically-excavated Buddhist, Jain, and Hindu monolithic temples in India's Maharashtra providence illustrates a spirit of tolerance characteristic of ancient India.

Holy Hindu shrine in an enormous, monkey-filled limestone cavern.

A cave's rare beauty held a cavernous secret hidden underground.

The second-deepest known cave on Earth, hidden in an extremely remote mountain region.

Largest cave system in Africa.

A beautiful geyser under environmental threat.