This former Masonic hall has been turned into an art project that uses even more metaphysical iconography.

A cave dug by a man directed by ghosts to find pirate treasure.

This upscale streetwear store is hidden behind a fake Snapple machine in the back of a deli.

A crumbling hospital from the 1850s on Roosevelt Island.

The oldest Italian café in Boston, this spot also serves as a veritable museum of vintage coffee ephemera.

A beautiful Victorian-era cemetery, complete with a miniature village.

An eccentric inventor's castle home with its own indoor weather system.

A morbid secret lies hidden within the beautiful walls of the Boston Athenaeum.

The only structure left with direct ties to the Salem witchcraft trials of 1692.

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

The hidden "ballroom" under a mansion lake was built by a criminal who ate a cyanide pill during his trial.

An ancient capital city that dates back 3,000 years old and harbors many surprises.

Most-visited and most-photographed slot canyon in the American Southwest.

On the Oregon coast there is a natural hole that seems to be draining the sea.

A collision of human error and natural geothermal pressure created this rainbow-colored geologic wonder.

An exquisite tunnel of cascading flowers.

A fiery crater has been burning in the Karakum Desert since 1971.

The intricate stonework of Machu Picchu serves as a testament to Incan engineers.

The ghost town left by the worst nuclear disaster of all time is being taken over by nature and urban explorers.

A huge musical instrument played by the ocean.

An emerald-green grotto just a short trip from Austin.

This astonishingly geometric cave has inspired everyone from Jules Verne to Pink Floyd.

During prohibition, corrupt city officials ran drinking dens under the streets of Downtown Los Angeles.

Scattered throughout London are ancient remnants of the city's former bounding wall.