Aqueducts built by the Romans mostly date to the Imperial period, though metropolitan Rome did acquire four under the Republic. Our knowledge of them comes from archaeology and literary. Archaeology is centered chiefly on the great bridges and arcades that form so prominent a part of the aqueduct network. For a full understanding, two qualifications must be noted. First, most of the length of the aqueduct conduits, being underground, remain invisible and hence can be found and visited only with the assistance and guidance of specialists. Second, within the city of Rome itself, the details of the urban distribution network remain uncertain and hard to follow, largely because of the modern city built on top. As for literary evidence, Vitruvius does give us an account of aqueduct building, but our chief source is Sextus Julius Frontinus who was appointed in 97 CE by Nerva as Commissioner of the Waterworks.