Fig. S2.

Map showing the potential Pb ore sources for the water distribution system of the Bay of Naples during the Roman period. Each colored pixel corresponds to a potential source of Pb ores. A computer query was conducted on our Pb isotope database (>6,800 data entries) (21) by comparing the Pb isotopic composition of the Piscina Mirabilis sample (Misenum), corresponding to the travertine deposit most affected by the Hercynian lead content in the Aqua Augusta water system travertine, with the mean value of Pb ores in each individual 0.25° × 0.25° cell. The confidence level of each of the identified pixels is shown in color with 2 (red), 4 (yellow), 6 (green), 8 (cyan), and 10 (blue) σ, where one σ is equal to 0.15 ‰ of the isotopic ratio value (for additional details, see ref. 21). Although one red pixel is identified in Bulgaria, the high density of pixels found in Western Europe (Spain, the Alps, France, Germany, and England) indicates that this part of the Roman Empire supplied the Pb ores used to construct the water distribution system in the Bay of Naples.