Hidden beneath the streets of London is a secret subway that only carried mail.

Mysterious rocks that ring musically when struck.

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

A crumbling hospital from the 1850s on Roosevelt Island.

The macabre collection of Jacques Sirgent, an expert scholar on the undead and all of their trappings.

The legendary building that needed ghostly approval before being built.

A 100-year-old purveyor of curiosities, curios, and kitsch featuring mummies, shrunken heads, and a Fiji Mermaid.

This structure among the trees was the result of divine inspiration.

High on a hill overlooking Los Angeles lives a world famous, high power conical Tesla Coil.

A treasure trove of artifacts, books, and artwork from the "golden age" of exploration.

A strange house built only at night, with rumored mystic inspiration.

The world's largest collection of witchcraft regalia.

The fairy tale castle of the "fairy-tale king."

A museum devoted to bringing to light forgotten or neglected histories through exhibitions, education and public programming.

Collapsing castle on the Hudson river was once the storage site of military surplus business.

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

The neon signs from Las Vegas' past find their final resting place at this unusual graveyard.

The sign at the garden gate reads: "These Plants Can Kill."

This iconic L.A. bookshop is housed in an abandoned bank—both symbolic and chic.

A huge musical instrument played by the ocean.

Non-profit time travel mart sells goods you would need for a trip through the fourth dimension.

A terrific purveyor of natural history objects and curios.

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

America's most famous museum of medical oddities is home to the remains of Albert Einstein's brain.