The subterranean "lake" below the Paris opera house inspired the Phantom of the Opera's lair.

Medieval museum built over Roman baths that features famous tapestries of a lady and a unicorn.

A 400-year-old church covered floor to ceiling in ex-voto offerings from the faithful.

The celebrated eyesore created by starry-eyed sweethearts.

A counter-culture bookstore like no other hides in plain sight in the streets of Paris.

France's national museum of scientific and industrial instruments.

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

A Parisian exterminator's shop whose storefront displays stuffed rats hanging from traps.

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

A series of abstract, silly sculptures represent the works of a great modern classical composer.

Once a train station along the Great Eastern Railway this vibrant coffee shop and bar is where members of London’s creative class mingle.

A recently restored Shoreditch landmark with ties to Jack the Ripper.

Historic church where a number of Shakespeare's actors are buried.

Scattered throughout London are ancient remnants of the city's former bounding wall.

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

Pop culture has helped to keep this iconic British building from being destroyed.

This hidden green space is the city's second largest conservatory.

This one has sailed five times as far as Sir Francis Drake's famous ship.

Black cat statues guard this temple-like Egyptian revival factory.

Rebuilt remains of a temple to Roman god Mithras.

Kings Cross Station pays tribute to its role in Britain's best-selling book series.

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

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

Hear the quietest sound from across the dome.