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

"Only the dead get off at Kymlinge."

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

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.

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

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

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.

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

One of the world's largest collections of pathological specimens.

A collection dedicated to the treatment of disease, held in London's oldest hospital.

A Victorian natural history and ethnographic museum with wonderful turn-of-the-century, science-book-esque evolution displays.

The anatomical-pathological collection of a man who changed surgery.

Catacombs turned marketplace.

The only university zoological museum in London houses extinct animals, bizarre natural history specimens, and a Micrarium of microscopic creatures.

A hands-off space for graffiti artists to hone and display their skills.

A subterranean ice warehouse that has lain hidden beneath central London for over a century is now open to the public.

Banksy gives themself artistic license with this piece of London street art.

Winding paths and ramshackle mausoleums in the oldest "park cemetery" in London.

This collection of illusory paraphernalia has been culled from the history of London's premier magician's club.

This South London cemetery holds the remains of more than 15,000 people.

This churchyard arbor is surrounded by hundreds of gravestones placed there by author Thomas Hardy.

A kaleidoscopic warehouse-maze of handmade neon signs.

This ornate Victorian marketplace was the setting for Diagon Alley and the Leaky Cauldron in the Harry Potter films.