This whimsical castle of grottoes was the labor of love of a French postman, and one of the greatest achievements of outsider architecture.

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

A glowing underwater cliff attracts marine life and divers in equal measure.

The oldest badlands in North America, Makoshika State Park takes it name from the Lakota words for broken land.

Dolphins, ancient clams and hundreds of waterfalls make this one of the world's top tourist destinations.

These millipedes glow not out of attraction, but as a not-too-subtle warning.

This natural gas vent certainly earns its name as the smallest volcano in Italy.

This expansive bridge in the Canadian wilderness was originally just built for friends.

A mysterious light floating in the trees of Arkansas may be a piezoelectric effect.

A massive work of modern art that appears to be a crop circle in the middle of the desert.

An obscure waterfall that looks like something out of a fantasy novel.

A failed piece of Orientalism has become the proud symbol of a community.

A once grand hotel is being retaken by the wilderness it once capitalized on.

Abandoned waterpark turned counter-culture mecca.

A religious-themed park filled with folk art to bring biblical stories to life that fell into disrepair and now being restored.

This abandoned amusement park was built on a site with a dark history.

These tombs were built into the side of a mountain so that ancient angels could reach the dead.

Striking occult iconography on a medieval German church.

A tropical paradise once home to space exploration and military installations.

Reclaimed by nature, the ruins of Doodletown, New York are remnants of a town consumed by a state park.

A hidden treasure of an abandoned city lies deep in the jungles of Campeche, Mexico.

One man's ambition created this psychedelic log home.

Despite its name this unique art piece was established to commemorate a tragic flood.

A castle built for love, and abandoned due to heartbreak.