400 million-year old limestone caverns nestled in the Ozarks served as shelter and a hideout for cultures throughout North American history.

The world's largest man-made underground caverns offer unparalleled underwater wonders.

Once an ambitious nature spa, these forest ruins now provide a serene monument to failure.

Less a museum than a bizarre fantasy world created by artists and engineers.

This empty Victorian manse on Staten Island is said to be haunted but is actually a real life crime scene.

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

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

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

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

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

A semi-scholarly approach to sex.

Elevated freight railway turned wildly successful urban park.

New York's oldest magic store.

This abandoned subway station beneath the Waldorf-Astoria hotel is allegedly used to secretly transport presidents.

Amid the bustle and noise of the Financial District hides a secluded garden oasis above the city streets.

These "anatomical machines" are only one of the many strange items in this Italian crypt.

A medieval weapon rumored to have been used by the brilliant poet himself.

The doodles of a genius line the walls of a secret room under the Medici Chapel, where they were hidden for nearly 500 years.

This Venice bookstore has resigned itself to constant flooding by keeping its books in bathtubs and boats.

The martyr's wax-enhanced skeletal remains lie preserved in a glass case in the Santa Maria della Vittoria in Rome.

The Pope’s secret escape route.

This hidden Art Nouveau courtyard is an opulent feast for the eyes.

The yawning maw of this pagan visage is said to bite off the hands of liars.

A master of trompe-l'oeil creates a stunning, tourist-fooling dome out of a flat ceiling.