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

Abstract dinosaur models in London.

London's smallest police station is barely the size of a phone booth.

Hear the quietest sound from across the dome.

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

Remains of Roman amphitheater hidden in a sleepy quarter of Paris.

Site of a classic 19th-century physics experiment to prove the earth is rotating.

Inside a former cabaret, seven floors of erotic art.

This iconic Parisian bookstore doubles as the "Tumbleweed Hotel" for traveling writers.

The vast, legendary catacombs hold secrets much stranger than stacked bones.

France's most famous cemetery, with some of its most curious tombs.

The celebrated eyesore created by starry-eyed sweethearts.

A Parisian cemetery where the elite and artistic are interred beneath an eccentric collection of tombs.

Vacant Beaux-Arts buildings that serve as reminders of a beach town’s glory days.

A world-class collection of mechanical musical instruments and automata.

A dark patch of the ceiling at Grand Central Terminal which was not restored is still stained brown by tobacco.

The first archaeological site discovered by aerial photography.

This historic railway museum may have lost its passengers but not its love for trains.

The US Navy's only submarine museum is also home to the world's first nuclear submarine.

A grand New York post office that is home to mailman Santas, scraps of postal history, and a pneumatic mail system.

The rotting skeleton of a shoreside fun fair that was destroyed by fire and storms still haunts the Brighton ocean view.

A pedestrian island in the middle of world-famous Times Square emits a strange (and purposeful) humming noise.

Elaborate tile work in this railway station tells the story of Portugal.

Built hundreds of years ago this downtown iron barrier once protected a much despised king.