The Panthéon

Originally built as a church dedicated to Saint Geneviève, the patron saint of Paris, this building has had more identity crises than any structure in Europe. Church, then revolutionary temple, then church again, then temple again — back and forth five times before the Third Republic finally settled the matter in 1885 by turning it into a secular mausoleum for France's greatest citizens.

The crypt reads like France's ultimate hall of fame: Voltaire, Rousseau, Victor Hugo, Émile Zola, Alexandre Dumas, Marie Curie (the only woman for decades, and the only person interred for achievements in two different scientific fields). When Hugo died in 1885, two million people joined his funeral procession through Paris — one of the largest gatherings in French history — and it was his burial here that cemented the Panthéon's purpose.

In 1851, physicist Léon Foucault hung a 67-meter pendulum from the dome to demonstrate the rotation of the Earth. The experiment was so elegantly simple it's almost annoying: a heavy brass ball swings back and forth, and over hours the plane of its swing appears to rotate because the Earth is turning beneath it. A replica still swings in the nave, though the original is at the Musée des Arts et Métiers.

The building itself is architect Jacques-Germain Soufflot's neoclassical masterpiece, inspired by Rome's Pantheon and London's St. Paul's. The dome nearly collapsed during construction because Soufflot's design was more ambitious than the available engineering allowed. Iron reinforcements were added after his death, and the building was finally completed in 1790.