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

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

Site of a classic 19th-century physics experiment to prove the earth is rotating.

This iconic Parisian bookstore doubles as the "Tumbleweed Hotel" for traveling writers.

A Parisian cemetery where the elite and artistic are interred beneath an eccentric collection of tombs.

The history of radiation is on safe display at the foot of a radiation science laboratory.

This statue was given to France by the United States to commemorate the centennial of the French Revolution.

Abandoned railway line circling the city of Paris.

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

An ancient Egyptian temple in the middle of Madrid, Spain.

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

France's most famous cemetery, with some of its most curious tombs.

A horde of stray cats rule in this secluded Parisian cemetery under a bridge.