This giant underground cave chamber can only be accessed through a small opening at the top.

A cave covered with bizarre stalactites of cloth.

Hide from the tourist hordes in these relatively unknown, and unguarded lava caves.

A once-glowing grotto at the bottom of Waikapalae Wet Cave.

This underwater cave in Madagascar is home to what is thought to be the country's largest fossil graveyard.

Largest cave system in Africa.

A hyper-remote desert cave dramatized in "The English Patient."

The massive cave church of Cairo's Zabbaleen community is one of the largest Christian churches in the Middle East.

A salty cave excavated by elephants and the location of a deadly disease vector.

This sacred, circular cave is lined with pools so clear and still that it's hard to tell where the water begins.

Veryovkina Cave is more than 1.3 miles deep—the deepest known in the world.

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

The second largest in the world, this massive underground void has its own subway.

The cave holds a spectacular array of stalactites and stalagmites, including one shaped like a giant human heart.

Ancient rock-hewn town that witnessed key events of Georgian history.

This neon lit Georgian cave claims to hold the boulder Prometheus was chained to.

Home of a 28-foot tall stalagmite known as the War Club.

Civil War dumping ground for the bodies of terrorized Union sympathizers.

A gargantuan crystal kingdom.

Inside this mountain lies several kilometers of breathtaking caves.

A massive underwater sinkhole made famous by Jacques Cousteau.

Sparkling, calcite-covered skeletons of children who may have been sacrificed to appease an angry god.

An ancient system of freshwater caves filled with animal bones, including that of species long-extinct.

Inhospitable caverns just beneath this island paradise offer insights into alien worlds.