World's first "penitentiary," meant to be humane, drove men insane.

Hidden beneath a bridge, this waterfall is known for the large amounts of graffiti painted on its base.

This pristinely abandoned filling station on the edge of a graveyard is populated solely by unsettling mannequins.

Visitors to this arboretum are greeted by three smiling giants crafted from sustainable materials.

The first "Hut" was actually a small brick building.

The doodles of a genius line the walls of a secret room under the Medici Chapel, where they were hidden for nearly 500 years.

Surprisingly aquamarine waters fill deep lakes formed at the end of the last ice age.

The quarry was abandoned over 100 years ago, after a massive ammunition explosion ended in tragedy.

It wouldn't be long after purchasing this farm, Brown would lead his siege on Harper's Ferry.

Built as a gift to the hottest tastemaker of the time, these hidden quarters remain an Art Deco showpiece.

The only museum with its curator—and her pets—buried inside.

These medieval skyscrapers dominate the skyline of this small Tuscan town.

The graveyard is filled with beautiful funerary art and boasts breathtaking views of Florence.

A slightly creepy gallery of plaster cast models in the same museum as Michelangelo's David.

This Art Deco fortress in upstate New York gives Manhattan's vaunted examples of the style a run for their money.

A reversed traffic signal that was once violently misinterpreted as a statement on British-Irish relations.

Dedicated to the art and science of glass in equal measure, this space is as much laboratory as museum.

Rummage through this sunken ship, but leave your scuba gear at home.

Abandoned rock quarry now a haven for mountain bikers and hikers.