The "Greatest Sewer" of ancient Rome is one of the oldest sewer systems in the world, and is still in use.

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

This massive stained-glass masterpiece is thought to be the largest Tiffany dome in existence.

No one knows how many bodies are in this vault in a Chicago public park.

This whimsical Oz-themed park had a wonderful impact on the surrounding neighborhood.

Partially submerged Buddha heads freckle Chicago’s neighborhoods.

A hot dog stand where the heads are just as hot as the dogs.

For decades, Puerta del Sol has been illuminated by a giant, guitar-wielding bottle of sherry.

The mysterious remains of a martyred teenage saint are venerated inside a small country chapel.

This influential Renaissance-era library was once the haunt of kings and monks.

This underground church is topped with a 500-foot cross that is seen by many as a beacon of fascist hubris.

This sweet-toothed bear searching for snacks has been a symbol of Madrid for decades.

Among the oldest structures in Madrid, these Moorish walls once guarded the medieval city of Mayrit.

A curious carving hides on a marble column in the Mosque-Cathedral of Córdoba.

Spain's unique Mosque-Cathedral has been a place of worship for both Muslims and Christians throughout its history.

A stone slab in Madrid marks the point from which the distances of Spain's national roads are measured.

This former charity hospital for elderly clergy houses some of Seville’s most treasured paintings.

The iconic boulevard is the perfect place to catch sight of a bicycling skeleton, a levitating Satan, or a chef with his head stuck in a pizza.

A vista of the city and sea from atop the little-known viewing deck on an unassuming Columbus statue.

The final resting place of the great navigator, or maybe his brother.

One of the few remaining ancient aqueducts described by Frontinus as 'the most solemn testimony of the Empire.'

A lush indoor jungle fills this bustling Madrid transportation hub.

Over two centuries old and counting, Madrid's massive botanical garden contains one of Europe's only desert environments.

Commonly acknowledged as the only public monument to the Devil himself.