The ruins of this Dutch castle sit atop a labyrinth of once-secret tunnels and passageways, dating back over 900 years.

Climb inside an iron crystal magnified 165 billion times its normal size.

Lavish royal estate punctuated with heated greenhouses.

Part modern art project, part traditional chapel, this see-through church is only substantial from the right angle.

This hidden Art Nouveau courtyard is an opulent feast for the eyes.

A mysterious chasm in the heart of the ancient Roman Forum, the Lacus Curtius was once believed to be a gateway to hell.

The underground sanctuary of a mysterious ancient cult.

The engineering wonder and pleasure dome of a much-maligned emperor still holds undiscovered secrets.

Hundreds of lucky felines haunt the ruins where Caesar was murdered.

An alchemist's "magic door" stands in the middle of a Roman park.

This domed temple was built in a hidden cave mouth as a refuge for sinners seeking absolution.

This private skyway is filled with art and was expanded explicitly so that Hitler could appreciate a river.

Over 50 exquisite hand-painted Renaissance maps decorate a Medici storage room.

The middle finger of modernity.

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

The oldest operating pharmacy in the world features 800-year-old recipes.

The secret medieval communication system will whisk your words above the noise of the crowd.

Two very green residential towers cleaning the air in one of Europe's most polluted cities.

Old batteries and frog skeletons reside in this lakeside monument to Alessandro Volta.

A lush indoor jungle fills this bustling Madrid transportation hub.

Located in Nairobi National Park, this animal orphanage is the oldest of its kind in Kenya.

Get tall with nature's lankiest beasts and their warthog friends at this Kenyan preserve.

A massive stairway that almost wasn't leads to one of Australia's most scenic vistas.

This trio of Australian rock spires may have inspired aboriginal legend or just modern tourism.