A tunnel once abandoned now thrives with graffiti art as an off-limits makeshift gallery.

A hip bar decorated with walls of golden skulls.

The feral felines of Roosevelt Island have a small army of allies and an abandoned hospital all to themselves.

Manhattan’s Battery Park has a whimsical carousel full of spinning, glowing fish.

Dodge prohibitionists and slide through the telephone booth in Crif Dogs.

This beautiful building in the heart of Manhattan's Financial District was empty for decades.

A crumbling hospital from the 1850s on Roosevelt Island.

Tucked in the back of this amazing antique store is an important piece of medical history.

La Monte Young and Marian Zazeela's "Dream House" will immerse you in an ever changing world of sound and light.

In this Parisian garden's grotesque former life as a zoo it fed exotic animals to the bourgeois and put humans on exhibit.

Original cabaret handbills, posters, and feline artwork are a purrfect highlight of the Museum of Montmartre.

A museum which takes visitors to the côté obscure of Paris.

Despite being considered a thoroughly American sport, basketball's oldest surviving playing field is found in the basement of a Paris YMCA.

This museum in a Paris apartment gives a unique look at the iconic French singer's life.

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

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

A subterranean shelter remains intact under a busy Paris railway station.

The largest medical and anatomical collection in France lies hidden from the public.

Paris' fabled purveyor of exotic taxidermy and natural history curios.

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

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

"In the Dark" Restaurant in Paris offers dinner in complete darkness.

France's national museum of scientific and industrial instruments.

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