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

Part modern art project, part traditional chapel, this see-through church is only substantial from the right angle.

Construction of Barcelona's iconic (but controversial) church is expected to be completed in 2026—a century after the death of its architect.

An abandoned cemetery lies in ruins as the surrounding forest swallows it whole.

This massive subterranean fissure is known as the "underground Grand Canyon."

A stunning, two-tiered waterfall made famous by poets, artists and the legend of a ghost dog.

Hear the quietest sound from across the dome.

The 953 windows undoubtedly make this the world's most beautiful screened porch.

The gorgeous library at Trinity College is home to remarkable treasures of Irish identity.

The hallucinogenic center of the ancient world.

The atmospheric road was never meant to be so creepy.

World's largest cave was only discovered in 1991, and now offers tours to the public.

Elevated freight railway turned wildly successful urban park.

A speakeasy takes on Chicago's drinking culture.

The walls and ceiling of this Polish church are decorated with thousands of skulls, with another 21,000 skeletons just below.

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

The loneliest island in New York.

An underground city of salt.

A Hawaiian stairway to heaven—that you can't climb.

An Escher-inspired library dedicated to the wonders of the human mind, yet closed to the public.

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

Stalactites and stalagmites in a beautifully lit cave.

A marvel of Byzantine engineering under the streets of Istanbul.

In all of the 29 years it was in operation, no prisoners ever escaped successfully... or so they claim.