Hidden within this iconic mountain are a set of World War II bunkers once used by German forces.

A burial vault located under Tiber Island in the middle of the Italian capital.

This delightful little museum is dedicated to only one thing: pasta.

Rome's semi-secret peephole vista is also a former Crusader stronghold.

Architectureal fantasy quarter of Rome.

This hidden Art Nouveau courtyard is an opulent feast for the eyes.

Hundreds of lucky felines haunt the ruins where Caesar was murdered.

An ancient tree that was once the favorite of a young Swedish royal.

A Stockholm metro station is home to an escalator that is not only the longest in Sweden, but among the longest in the world.

Sound travels strangely in this underground whispering gallery.

Stockholm's narrowest alley is barely over two feet wide.

A man has revenged his true love's betrayal through tears and stone genitalia.

Street artists transformed this abandoned industrial neighborhood into one of Europe's largest graffiti exhibitions.

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

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

This Stockholm light tower can be controlled by anyone with a smartphone.

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

The library's graceful rotunda is like a bibliophile's temple.

It houses the remains of a 17th-century version of the Titanic.

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

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.

Amsterdam's Red Light District leaves little to the imagination.