A thin pane of glass keeps visitors from sliding off this futuristic skywalk.

Longest road tunnel in the world features an air treatment plant and the illusion of daylight.

There's a 12-foot-tall replica of the Washington Monument hidden under a manhole nearby.

California's own medieval Italian castle, authentic down to the 300-year-old iron maiden in the dungeon.

The medieval castle built as headquarters for the Pope's secretive Knights Templar.

Japan's largest collection of Western-style mansions in the port city of Kobe includes a recreation of Sherlock Holmes' 221B Baker Street quarters.

Formerly secret, precariously perched boulders.

A statue commemorate Tommie Smith and John Carlos's brave protest at the 1968 Olympics, a watershed moment for civil rights.

The "world's smallest volcano" has been used to store meals and dispose of dead bodies.

A bust of Franz Kafka spins in pieces, reflecting the writer's inner torment.

The sci-fi villain is a little-known inhabitant of the U.S. capital's largest cathedral.

The only home that Alexander Hamilton ever owned has a history almost as troubled as his own.

Sacked by Vikings, blessed by William the Conqueror, dealt a fatal blow during the French Revolution—these ruins have seen it all.

An otherworldly restaurant perched on the edge of a jagged cliff next to a medieval holy city.

Dramatic sea stacks adorn the coast of this small village on the Adriatic coast.

A huge work of Burning Man-inspired outsider art on the grounds of a California winery.

An expansive complex of subterranean tunnels is all that remains of one of Europe's most formidable medieval fortresses.

The world's brightest bioluminescent bay.

A million and a half tons of earth were used to create one of the world's largest land sculptures.

A remote candy-striped mountain in the Peruvian Andes straight out of the pages of Dr. Seuss.

The country's oldest continually-operated mini golf course lies hidden in plain sight, eclipsed by one of D.C.'s most popular tourist attractions.

A spiral staircase twists to the top of this unusual Czech lookout tower.

Amid the bustle and noise of the Financial District hides a secluded garden oasis above the city streets.

Bring your own cardboard to the slippery slopes of concrete hidden in a neighborhood park.