The buildings are constructed in travertine, brick, lime, pozzolana, and tufa. The complex contains over 30 buildings, covering at least a square kilometre (250 acres, an area larger than the city of Pompeii), of which much is still unexcavated. Villas were typically sited on hilltops, but with its fountains, pools and gardens, Hadrian's villa required abundant sources water, which was supplied by aqueducts feeding Rome, including the Aqua Anio Vetus, Aqua Anio Novus, Aqua Marcia, and Aqua Claudia. To avail themselves of those sources, the villa had to be located on land lower than the aqueduct.