The beautiful baroque library of the Hapsburg empire.

A wondrous old library overlooking the nation's capital.

A 19th century artistocrat's mansion, turned into a library, hidden in a modern library.

A little-known subterranean street runs under some of the best-known buildings in Chicago's River North.

A tour of underground Berlin takes you through WWII air raid shelters and tunnels used to smuggle people out of East Germany.

Long-abandoned trolley tunnels just a mile away from the White House are turning into an art space.

Travel through the hidden underbelly of the city in colorful tunnels filled with art and history exhibits.

A labyrinth of ancient tunnels hidden below the city holds the ruins of 2,500 years of history.

There's a labyrinth of medieval chambers hidden deep below a 15th-century pub.

This underground transport hub doubles as an art gallery.

The gorgeous entrances into the city's underbelly look more like grand halls than subway stops.

Air-raid tunnels hidden beneath this opulent hotel provided shelter and an escape route—and one of the world's longest slides.

Located more than 600 feet beneath sea level, this tunnel boasts the world's first undersea roundabout.

Would you spend the night in Japan's deepest train station?

Crypt mausoleum in cathedral's basement contains relics of third century Roman martyr, Saint Vibiana.

The legendary building that needed ghostly approval before being built.

Wondrous sandstone hoodoos where Native Americans collected clay for pottery, 9,000 years ago.

The only building in Louisville (hopefully) that contains two ancient holy skeletons.

An enormous pair of straw sandals woven in the traditional way—to keep demons at bay.

The only Chinese temple in Mumbai.

A Zoroastrian funerary tower where excarnation rituals are carried out is threatened by the area's dwindling vulture population.

Singapore's landscaping project is home to 18 incredible Supertrees.

Singapore's last surviving traditional village is hidden among giant skyscrapers and modern infrastructure.

This Vietnamese hell cave lets visitors take a walk through a devilish underworld full of demons.