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

The fairy tale castle of the "fairy-tale king."

The intricate stonework of Machu Picchu serves as a testament to Incan engineers.

An Italian crevasse filled with abandoned buildings gives visitors a look at the world without humans.

An exquisite tunnel of cascading flowers.

The mysterious, fantastical woodland inspiration for "The Lord of the Rings."

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

Experiencing Guatemala's sensory-exploding church is a blunt reminder of European influence.

Central America's deepest lake is actually a volcanic caldera, surrounded by three volcanoes.

Volcanically heated hot springs.

Hidden deep in the Guatemalan jungle is a turquoise paradise of natural pools, caves, and waterfalls.

These aggressively sustainable art homes look like something out of 1970's science fiction.

A multi-storied adobe complex has been inhabited for more than a thousand years.

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

Seemingly grown on some other world, these New Mexico rock formations look like a disused science fiction set.

The second-largest cave chamber in the world was discovered in 1898 by a 16-year-old and a friend known as "Pothead."

A museum dedicated to the alleged Roswell Incident.

Explore the largest pure gypsum deposit in the world, and go dune sledding while you're at it.

One man has carved a number of natural New Mexico caves into psychedelic sandstone temples.

An immersive environment that is part haunted house, part choose-your-own-adventure, and part jungle gym.

An emerald-green grotto just a short trip from Austin.

On the Oregon coast there is a natural hole that seems to be draining the sea.

A huge musical instrument played by the ocean.

Secret Swing.