Once the path of a railway tunnel, now home to thousands of glowing insects.

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

The old Niles Canyon Aqueduct in the San Francisco Bay Area is a bemusing and secret trail through nature for wayward travelers.

A subterranean Holocaust refuge within one of the world's longest cave systems.

The medieval network of canals hidden underneath the city is opened to explorers each year.

A room hidden behind a swinging shelf in the children's section.

This lower Manhattan bar is set up in an old bank vault that rests beneath a skyscraper that hides even more historic splendor.

A legendary speakeasy bookstore returns from the brink.

Hundreds of hidden passageways weave through the old quarters of Lyon.

A Pythagorean funhouse that tries, successfully, to prove that math is the coolest thing ever.

A hands-on traveling collection of the innovative devices of yesteryear.

This former Norwegian sailor's church is slowly being taken over by ramshackle robot musicians.

A DIY art gallery/music venue/lecture hall dedicated to video game culture.

The one-stop shop for all of your superhero (or evil arch-nemesis) needs.

A miniature rainforest is growing inside of a Midtown Manhattan office building.

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

A terrific purveyor of natural history objects and curios.

An abandoned castle owned and ignored by Masons carries a cursed history.

A collection of haunted artifacts chronicles the career of the world's most famous paranormal investigators.

A snapshot of the city's fascinating voodoo culture.

The abandoned zoo enclosures offer a perfect place to experience the other side of the bars.

The inspiration for H.P. Lovecraft's horror stories and Batman's Arkham Asylum is now a condominium complex.

A toxic ghost town sitting atop a massive coal fire.

A museum dedicated to the quirky and colorful world created by Springfield's favorite son.