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

A student driver's nightmare.

This London pub hides a peculiar secret: a recreation of the rooms shared at 221b Baker Street by Sherlock Holmes and Dr. Watson.

The popularity of Sherlock Holmes led to the creation of his fictional address, turning the orderly London street numbers askew.

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

Hidden beneath the streets of London is a secret subway that only carried mail.

A 12th-century fortress containing some of Berlin's oldest buildings.

The world's first clock to use set theory as a measure of time required the invention of a new light bulb just to maintain its obtuse measurements.

Where once there was a train station bathroom, there is now this German burger joint.

Berlin's old airport.

The abandoned remains of this Victorian brewery hide crumbling bowling alleys and rusting industrial gear.

This Berlin island is home to a flock of free-roaming peacocks, as well as a historic ivory folly.

Blitzkrieg bop your way through Berlin's Ramones Museum.

A tree-like building perches on its trunk in the Steglitz neighborhood of Berlin.

This futuristic German spa allows visitors to float in a pool of salt water while zoning out to underwater techno.

Started as a kitchen garden, now the second largest botanical garden in the world.

The most visited, and possibly most controversial, museum in Germany.

The largest free-standing aquarium in the world resides in a hotel lobby in Berlin.

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

Rome's ancient Etruscan rival, now a park full of ruins.

2300-year-old Roman road connecting the empire.

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

A master of trompe-l'oeil creates a stunning, tourist-fooling dome out of a flat ceiling.

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