A 140-year-old castle with underground caves hidden in San Francisco.

Bring your own cardboard to the slippery slopes of concrete hidden in a neighborhood park.

A huge musical instrument played by the ocean.

A crucified and bearded female saint.

The last municipal pneumatic post system in the world was in operation for over 100 years, and may run again.

The hidden lair of an alchemist rediscovered in a 2002 flood.

Two stunning 17th-century libraries and a cabinet of curiosities.

The history of occult science is turned into a creepshow at this lurid Prague tourist attraction.

A bit of retro transportation flair preserved in the world's largest department store.

A staggering collection of photographs depicting medical, social, and historical change.

Revolutionary War casualties fill the only 17th century structure remaining in Manhattan.

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

An accidentally hidden library containing the ephemeral history of New York.

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

An unassuming secret door once led to a legendary watering hole.

Walk over a 150 year old manhole.

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.

Hidden in an ordinary Midtown building is a one-room museum dedicated to the greatest escape artist.

Mysterious 500-year-old tapestries depict a unicorn hunt.

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

This cemetery has graves dating back to the 17th century, including the city's oldest carved tombstone and an ominous cryptogram.

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

A hip bar decorated with walls of golden skulls.

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