Church of Saint-Étienne-du-Mont

Workers are still discovering medieval stones beneath your feet here at Place Sainte-Geneviève, because this Renaissance masterpiece sits directly on top of a 6th-century abbey crypt. Saint-Étienne-du-Mont took a staggering 134 years to build, from 1492 to 1623, which is why you're seeing this wild architectural cocktail that exists nowhere else in Paris. Look up at that façade – it's actually leaning slightly leftward due to the ancient ground shifting beneath it, though most visitors never notice. Those three distinct pediments tell the story of France's architectural evolution in stone, with Gothic flames morphing into Renaissance geometry as your eyes move upward. Step inside and you'll find yourself facing the only surviving rood screen in all of Paris – that ornate stone lacework suspended between the nave and choir that somehow escaped the Revolution's hammers. Behind it sits the oldest functioning organ in the city, still hand-carved from 1631, while beneath the altar lie both the relics of Saint Geneviève, who convinced Parisians to fight off Attila the Hun in 451, and the tomb of mathematician Blaise Pascal, who invented probability theory right here in this neighborhood. This is where medieval mysticism meets Renaissance reason, literally built one century on top of another.