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

Abandoned railway line circling the city of Paris.

This iconic Parisian bookstore doubles as the "Tumbleweed Hotel" for traveling writers.

Continuing the tradition of the dime museum in style.

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

Northern Ireland's Brobdingnagian stepping stones.

A layered tribute to the work of mosaicist Isaiah Zagar.

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

The only theatre in the world constructed specifically for sound movement.

Wondrous sandstone hoodoos where Native Americans collected clay for pottery, 9,000 years ago.

An immersive environment that is part haunted house, part choose-your-own-adventure, and part jungle gym.

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

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

A small island less than half a mile from Venice is a forbidden island with a dark and twisted past.

Enormous lake at the bottom of a unique cave system.

This shop and gallery is a modern-day cabinet of curiosities, unassumingly positioned along an urban thoroughfare.

This ornate Victorian marketplace was the setting for Diagon Alley and the Leaky Cauldron in the Harry Potter films.

Famously misunderstood wonder of the ancient world

The birthplace of an eccentric alchemist and possible inspiration for the fictional reanimator of monsters.

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

Site of a former hidden treehouse in New Orleans.

Try not to get slimed in Tribeca.

Macabre astrological automaton clock dating to the late 1400s.

An underground mansion with fruit-bearing trees and grapes growing beneath the surface of central California.