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

This sculpture by artist Fritz Keonig survived the 9/11 attacks and now stands as a monument to the victims.

An uber-secure, windowless tower of doom in the center of Manhattan is an NSA spyscraper.

Manhattan's lighthouse, erected at the insistence of the unsinkable Molly Brown.

A beautiful and abandoned New York subway station from 1904, complete with chandelier.

There’s a secret bunker inside the Brooklyn Bridge, but the city won’t say where.

An odd, ancient window that may have once been part of a brutal prison is embedded in the side of New York's Police Plaza.

The world’s oldest and biggest bookstore stocking only mystery, crime fiction, espionage, and thrillers.

Once the tallest building in the world and the site of "the highest dinner ever held in New York" this building now holds the most expensive penthouse in the city.

A scattered trail of street art adds a pop of color to the city's stark structures.

The elaborate clockwork contraptions in this Scottish gallery create small worlds of whimsy and horror.

"Respectful to the dead, safe and sanitary to the living, dedicated to the Genius of Memory..."

The Argonaute submarine was the French navy's pride and joy in the 1950's.

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

This charming cobbled backstreet is the most Instagramable street in Paris, much to the residents' annoyance.

A curious collection of locks and keys.

An odd museum dedicated to French fakes.

The Sun King's military models, saved from the Louvre.

Architect Le Corbusier built his friend a cubist house to match his impressive collection of cubist art.

BDSM, 1930s Style.

The floorboard scrawling of a tragic madman is on public display outside a Paris hospital, much to the dismay of its opponents.

This off-limits room in the National Library of France is home to a system of pneumatic tubes, once used to bring books to readers.

Museum on French naval history, with magnificent figureheads, a historic diving suit, and a boat used by Napoleon.

This life-size replica of the Statue of Liberty's flame is also a tribute to Princess Diana by default.