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

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

A detailed pachyderm supports Rome's smallest Egyptian obelisk.

This subterranean shrine beneath a bike rental store celebrates the iconic Italian scooter.

A wunderkammer of curious objects and intriguing art nestled within a hub of ancient Roman sites.

One of the most important yet well-hidden museums dedicated to the history of medicine.

One of several skulls claiming to belong to the beheaded St. John the Baptist is on display at this Roman church.

Architectural monsters are devouring this palace's every door and window.

The final resting place of the poets Shelley and Keats.

The skull of the patron saint of lovers lies in the Basilica di Santa Maria in Cosmedin—maybe.

At the crypt of St. Mary of Eulogies and the Dead you are left alone to ponder mortality among piles of skulls.

The underground sanctuary of a mysterious ancient cult.

The only "Egyptian" pyramid in Europe and the legendary tomb of Remus.

The crypts of Capuchin friars decorated with the bones of over 4,000 friars, including an entire "crypt of pelvises."

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

An unassuming store hides relics of its former life as the jazz club that hosted some of the best black performers of the 1920s and '30s.

In one Chicago park, a trio of werewolves are waiting to devour their fallen brethren.

A century-old network of freight tunnels underneath the city's historic business district.

A Museum of Holography frozen in 1974.

What was once one of the country's largest confectionary plants is now an abandoned ruin, but is still pretty sweet.

Not to be confused with the iconic New York City skyscraper, this Chicago building is a vibrant hub for artists.

Small and slightly hidden, the American Writers Museum is preserving the legacy of America's finest authors.

This surprising gem is a world-class archeological museum hidden at the University of Chicago.

A haunting statue called "Eternal Silence" has quite the reputation among visitors with active imaginations.