At the beginning of the second century AD, Rome was a city of perhaps a million inhabitants. A network of aqueducts and lead pipes distributed water from the Tiber River throughout the city. Because the pipes were made from Pb brought from elsewhere in Europe, their isotopic composition differs slightly from the Pb naturally present in Tiber river water. Hugo Delile, Francis Albarède (École Normale Supérieure de Lyon and Rice University), and colleagues have now measured Pb isotope ratios of sediments in the ancient Roman harbor at the mouth of the Tiber. Their data, as seen in the figure, show isotope ratios rising and falling in concert. Higher isotope ratios, marked by blue shading, represent higher levels of anthropogenic Pb leached into the water from the city water pipes. The dates along the right edge of the figure are derived from radiocarbon dating of organic matter and are accurate to within 100 years. Changes in anthropogenic Pb levels can be tentatively related to events in Roman history. The gradual decrease centered around AD 250 suggests that the water distribution system was falling into disuse, perhaps due to a decrease in urban population during the Roman Empire’s long decline. The Byzantine Empire’s conquest of the Italian peninsula and repair of the Roman aqueducts in the middle of the sixth century is consistent with the spike seen in anthropogenic Pb around AD 500. The sharp drop in anthropogenic Pb around AD 800 could be related to the sack of Rome by Arab raiders in AD 846 or to major flooding in AD 856. Geochemical analysis of more cores in more locations will help to refine the timeline and to link the data to recorded and unrecorded historical events. (H. Delile et al., Proc. Natl. Acad. Sci. USA 111, 6594, 2014.)