Inside a former cabaret, seven floors of erotic art.

A curious collection of locks and keys.

This recessed garden protects thousands of mountain plants by creating its own microclimates.

Two awe-inspiring globes from the reign of the Sun King hang from the ceiling of France's national library.

A modern-day Lascaux cave underneath Paris.

The curious underground history of keeping Paris clean.

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

The first green space constructed on an elevated viaduct.

The water-powered carriages are long gone, but for over a century this funicular railway has been the fastest route to the highest point in Paris.

Egyptian Revival movie palace abandoned for decades and restored in 2013.

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

Behind the pink curtain, Au Bonheur du Jour, an art gallery of erotic archives.

A surrealist timepiece by the artist of melting clocks.

This Parisian café sticks almost exclusively to an ethos of reuse.

A scandalous Art Nouveau collaboration that set Paris all atwitter at the turn of the century.

Abandoned railway line circling the city of Paris.

This massive library holds what was once the largest book collection in the world.

A miniature village lies nearly forgotten in the forest by the side of a major highway.

This whimsical stacked cabin looks like something out of a children's book.

New York's unstoppable progress turned this cemetery into the smallest burial ground in the city.

Hidden in plain sight on a pair of Manhattan subway tunnels are a pair of public instruments that sing to each other.

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

The installation’s official name, “You Who Are Getting Obliterated in the Dancing Swarm of Fireflies,” says it all.

Under a glass sidewalk lie the remains of some of Manhattan's oldest buildings.