Skulls from the plague of 1656 and World War II bombings are united by a cult dedicated to caring for the ancient dead.

This massive underground salt mine now holds a sci-fi theme park in its depths.

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

Michael Jackson got Bad and Crocodile Dundee walked on heads in this iconic Brooklyn subway station.

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

Montreal's colorful subway station shows its support of the local LGBT+ community.

A 90-year-old underground water reservoir offers a more industrial kind of tranquility.

Welcome to old Florida, where a 1940s mermaid show is still enchanting visitors.

Wild lights and sound effects turn a walk through some caves into an eye-popping 1960s tourist attraction.

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

A massive cave complex boasts some of the clearest water (and most charming bats) in the world.

The beautiful tunnel that ran from the lobby to Penn Station is still hidden underneath 34th Street.

Beautiful mosaics cover the first stations opened on New York City's newest subway line.

A Texas cave network is home to some of the world's most spectacular-looking and bizarrely named mineral formations.

Hundreds of glow-in-the-dark objects light up a museum in a historic zinc mine.

A cave 150 feet beneath Kansas City is the perfect climate for growing 10,000 delicate orchids.

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

The secret room where early Bolsheviks cranked out propaganda fliers on a smuggled printing press.

Colorful lights and mirror-like brine pools create trippy illusions within the historic mine.

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

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

The entrance to this sinisterly named tunnel hides behind a Tim Hortons.

Stockholm's deepest subway station is also a stunning ecological wonder.

Why have you never heard of the second most popular tourist site in New York?