The lead pipes of the Roman Empire distributed water from kilometers-long aqueducts (like the one above) all throughout their cities. And they did something else remarkable, too, according to a new study: They created a historical record of the cities they served. As water flowed through pipes and into harbors, it carried traces of lead, which eventually settled into harbor sediments. Researchers can now use cores of these sediments to make out when the ancient Romans switched their water delivery systems, taking advantage of the fact that the proportion of lead atoms with different weights changes depending on where the lead ore was mined. Studying sediments from the harbor of Naples, Italy, scientists found a sudden shift in sediment lead around 79 C.E., coinciding with the famous eruption of nearby Mount Vesuvius. Volcanic ash could have clogged the pipes, or ground motion could have damaged them, forcing the Romans to replace them with lead from a different source, according to a study published today in the Proceedings of the National Academy of Sciences. The sediments also revealed that over time, more and more of the pipe network came from sources distant from Naples. That suggests an expansion of the network of lead pipes that brought water to individual buildings—but only until the 5th century C.E., when economic collapse and other troubles put the region’s aqueduct out of commission.