Built in 1870 at the bridge that connected Buda and Pest, this spectacular incline railway was almost lost forever during World War II.

A vast Communist-era exhibition space in the Bulgarian capital.

This ancient theater, restored in the 20th century, lives on as a venue for dramatic and musical performances.

A striking symbol of the Bulgarian capital.

Also known as the Square Colosseum, this building is a prime example of fascist-era rationalist architecture.

This bridge was constructed from the rubble of the infamous Bastille.

The ruins of a Duke's bucolic dream, where camels once roamed alongside "Dutch" windmills and faux-Italian vineyards.

19th century pendulum and a clock restored by a rogue group of guerilla artists.

An "inside out" building in the historical center of Paris, featuring Europe’s largest modern art museum.

This ancient Egyptian obelisk still bears the wounds of World War I.

A mysterious shape catches the eye in this 16th-century painting.

More than a million titles fill the labyrinthine shelves of this prestigious private library.

A labyrinth hides in plain sight at each of the city's 270 Tube stations.

The London pub overlooks where the Mayflower was moored before setting sail for the New World.

Mary Kelly was last seen here before she was murdered by Jack the Ripper.

This massive collection of British military artifacts includes the heartbreaking personal letters of the men in the trenches of WWI.

Hidden behind a wall for decades, this masonic lodge was only rediscovered during renovations.

Hear the quietest sound from across the dome.

In this cobwebbed old operating theater, you can practically hear the screams of the unanesthetized patients.

Eighty million natural history specimens call this gargantuan museum home.

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

A 1,215-foot tunnel transports pedestrians beneath the River Thames.

One part local history, 99 parts gruesome haunted house.

A student driver's nightmare.