Roman-Germanic Museum, Cologne

Cologne began as Colonia Claudia Ara Agrippinensium, founded in 50 AD and named for the empress Agrippina, who was born on the Rhine and had it elevated to a Roman colony. It grew into a major city of the empire's northern frontier. In 1941, workers digging an air-raid shelter beside the cathedral struck the vast Dionysus Mosaic, a 70-square-metre floor of a wealthy Roman townhouse, perfectly preserved under the medieval city. The Roman-Germanic Museum was later built directly over it so the mosaic remains where it was laid nearly 2,000 years ago. The museum also holds the towering reconstructed tomb of the legionary Poblicius and Roman glass of astonishing delicacy.