A disquieting Aztec sculpture displays hundreds of stone skulls representing the victims of human sacrifice.

This macabre masterpiece of a mural is one of Francisco Goya's "Black Paintings."

These official units of measurement—including chains and perches—lie hidden beneath tourists' feet in a popular London hub.

These spectacular, symbolic serpents lie within the shadow of the Great Temple.

The legendary bathing site of a Moorish queen is actually part of a Roman fish farm.

A curious memento mori hangs in the magnificent Cathedral of Segovia.

This enormous censer uses a pulley system installed over 400 years ago.

A delightful museum devoted to the history of books.

This small museum tucked within a subway station holds pieces from Madrid's past.

Charles Dickens described the concert room in this amazing Victorian hall as "the most perfect room in the world."

This immense scene of a cosmic battle between a jaguar and serpent illustrates the Aztec concept of the duality of life.

A plaque claims this artwork outside a children's hospital was used in the 1998 Quidditch World Cup.

This memorial, built by fascist dictator General Franco in 1960, has been repurposed by the people of Catalonia.

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

There's a labyrinth of manmade tunnels stretching for over an acre beneath Bristol.

The Swedish flag over this small naval fort signals that the nation is at peace.

England's steepest street has such a drastic incline that locals tie their parked cars to lampposts when it gets icy.

The abandoned funicular tunnel was a secret base for the BBC during World War II.

The remains of the second-oldest zoo in Europe can be seen within these library gardens.

Stockholm's tiniest statue is of a small boy looking at the Moon.

It is said that each of the white stones in the red facade of this medieval house represents the head of a murdered Swedish noble.

Five million people are laid to rest here, more than the population of Madrid itself.

The castle still holds traces of the abandoned Roman fort it was built atop.

A botanical garden full of thought-provoking contemporary sculptures.