Partially collapsed lava tube and cave system complete with concert hall, underground pond, and unique albino crabs.

Louise Bourgeois' 30-foot-tall steel spider sculpture.

An abandoned saucer-shaped monument in the Bulgarian mountains.

The first tourist attraction to offer tours for interstellar tourists.

A late sculptor's last project was to turn this abandoned cement factory into a castle-themed amusement park.

The world's tallest church tower reveals a spectacular view to anyone up to the task of climbing the daunting staircase.

Underground monastery and fortress.

Medicinal spa created with the discharge from a geothermal energy plant.

The largest ice cave in the world is thought to be the entrance to hell according to Austrian legend.

The legendary cliffside castle of three-time Voivode of Wallachia, Vlad the Impaler.

A unique underground structure dating back to the 1st century BC.

Stalactites and stalagmites in a beautifully lit cave.

Fire meets ice at the southernmost volcano on Earth.

Spring that emits water and fire at the same time is said to have remedial properties, and sparked its own religion.

The subtle interplay of fire and water is hard to ignore at this remote Jamaican spring.

Michael Heizer's masterwork in the Nevada Basin.

This everlasting lightning storm may be the world's largest generator of ozone.

Men in rubber-soled boots serve as your "brakes" on this toboggan ride Hemingway described as "exhilarating."

A fantastical group of cliffside stairways, bridges, and twisting paths carved out of stone provide a surreal coastal hike.

A breathtaking canyon offers stunning views and frightful pathways through an ecological paradise.

This giant boulder is pinched over a 1000-meter abyss by two mountains.

A Spanish artist builds his own castle.

Visitors dip into a variety of pools of what was once ice.

400 million-year old limestone caverns nestled in the Ozarks served as shelter and a hideout for cultures throughout North American history.