The largest catacomb system in the world.

This attraction was abandoned after the Chernobyl disaster.

The ghost town left by the worst nuclear disaster of all time is being taken over by nature and urban explorers.

One of the oldest continuously-operated gay bars in the United States is allegedly haunted by Truman Capote and Tennessee Williams.

Large Hadron Collider and the Birthplace of World Wide Web.

Small Easter eggs are hidden in the ventilation holes of the city's newest trains.

This macabre masterpiece of a mural is one of Francisco Goya's "Black Paintings."

A completely underwater art museum sends a stark message about humans' impact on the environment.

A golden egg-shaped sauna in the Arctic tundra.

It is said that each of the white stones in the red facade of this medieval house represents the head of a murdered Swedish noble.

The world's first ice hotel is still one of the most elaborate and beautiful.

A rare surviving Victorian greenhouse designed specifically to house giant water lilies.

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

A hidden bathhouse in one of Stockholm's oldest and busiest neighborhoods is open to anyone who can find it.

Dine on food tied to Viking history and legend at this subterranean Stockholm spot.

Sweden's first nuclear reactor, R1, can be visited 25 meters underground.

An illuminated manuscript comprising the life's work of a lone monk, inexplicably decorated with a portrait of Satan himself.

Stockholm's decidedly unique public transit system has doubled as the world's longest art gallery for decades.

This New England covered bridge is home to the tale of a jilted lover known to haunt the area.

This pair of gilded girls atop an Art Deco building in Copenhagen were built to function as a weather indicator.

Climb to the top of its spiral spire for spectacular views of Copenhagen.

A little-known cafe in an old copper bridge tower with 360-degree views of the harbor.

A 19th-century subterranean reservoir repurposed as a contemporary art cave.

One of the last of the great 1920s movie palaces is a Floridian Greco-Spanish-Persian dreamscape.