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

One of the world's steepest streets seems to have houses sliding down it.

Cat art as far as the eye can see.

A floating cat sanctuary on an Amsterdam canal.

A series of five defunct panopticon prisons in Cuba.

A dangling church built over a Colombian gorge.

A sacred lake composed of over 300 separate pools of highly concentrated minerals.

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

Here, at the "world's longest art gallery," you'll find forty miles of ancient petroglyphs and pictographs.

A 30-acre garden inspired by the principles of modern physics.

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

Once the home of a hermetic monk, this cave now houses a small church.

A palatial sculpture park was built illegally and in secret by a self-taught artist over 36 years.

Natural cathedral designed by Italian artist Giuliano Mauri.

More than 200 statues filled with cryptic symbols give this park spot an aura of horror vacui fantasy.

Thousands of exquisitely carved stone water storage wells, hundreds of stone steps fill this exquisite medieval stepwell.

The central mosque of this Mali town is entirely built from mud.

The world's second-largest excavated hole.

A fifth-century temple improbably built into the side of a cliff.

A serpentine highway twists and turns over the treacherous Norwegian Sea.

Believed to have once stored either human remains or rice wine, thousands of mysterious stone jars lie in ruin.

Rock-cut caves and 7,000 Buddhist sculptures carved into the nearly sheer face of a sandstone mountain.

Houses are tucked between, on, and underneath giant boulders.

This abandoned, turn-of-the-century astronomy hub is now the subject of more photographs than it ever took.