This picturesque Umbrian city has a subterranean side that has long remained hidden.

Abandoned for decades, trains still pass through the now-restored remains of one of Madrid's first metro stations.

This well-preserved Cistercian Abbey in a romantic pastoral setting has inspired artists like the poet Wordsworth.

This North Carolina museum is keeping the pinball craze alive, with a little Space Invaders on the side.

Few other hotels can rival the grandeur of the Castle in the Rockies.

An indoor miniature village and labor of love for Laurence Geiringer, a model railroad enthusiast.

A breathtaking bridge that crosses a vast gorge over a river's torrents, and has a tale of a devil architect attached.

A mecca for India's Jains.

A pair of large white marble lions guard this stretch of the Mojave Desert, and it is utterly mysterious how they got there.

The remains of a 19th century spring, and the site of an infamous unsolved murder.

What may be America's largest classic video game arcade lurks inconspicuously in a Chicago suburb.

An architect's childhood fantasy treehouse made real.

Exquisite wall sculptures carved in secret by an early quarryman in an obscure corner of the Paris Catacombs.

The abandoned "Ghost Mansion" was left to decay in the mountains of Northern Italy.

An increasingly eccentric apartment building, topped with winged beasties.

Hidden in a grotto 26 feet below Nottingham's oldest hotel, this stylish subterranean drinking den is notoriously hard to find.

An abandoned railroad tunnel is now filled with bioluminescent bugs.

Madrid's romantically ornate crystal palace has been offering shelter to plants and art for over a century.

At the "Garage Mahal" in Houston, car culture is about more than just driving.

Thirty creepy ghosts now inhabit this decaying 14th-century church.

There's nothing creepier than a cemetery next to a clown motel.

Natural stone arches form a walkable "cathedral" at low tide, then disappear entirely when the water returns.

The summer home of the largest colony of bats in the world.

Perched on a spindly rock spire in a Greek archipelago, this lighthouse would make a perfect wizard's tower.