The subterranean "lake" below the Paris opera house inspired the Phantom of the Opera's lair.

An overlooked marble is the only surviving in-place 18th century example of the measurement that changed how we define distance.

Exquisite wall sculptures carved in secret by an early quarryman in an obscure corner of the Paris Catacombs.

A subterranean shelter remains intact under a busy Paris railway station.

Down a cobblestone alleyway is a museum dedicated to Romanticism, with a secret garden.

Abandoned railway line circling the city of Paris.

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

An enormous, inside-out glass globe built in 1935.

These old stone ruins in the Oregon wilderness were once bathrooms and are steeped in legends of murder.

A crumbling hospital from the 1850s on Roosevelt Island.

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