The large Roman cisterns are considered a true heritage of Augustan hydraulic art, as well as an ingenious idea of Vitruvius. It was almost certainly decided to create what are commonly called purifying or liming pools to meet a water need that could not otherwise be met. The extensive underground complex, dating back to the end of the 1st century BC, is unique in Italy in terms of size (about 2200 square meters). For storage and subsequent distribution, three tanks were built, located on Girfalco, in the current largo Temistocle Calzecchi Onesti and in via degli Aceti, which were obviously at different heights. The first one is currently not visitable because it is completely buried, while the second one, known as the small cisterns, is. Its capacity is smaller than that of the tank located in via degli Aceti, which, however, structurally does not present significant differences compared to the smaller one. The interior consists of thirty rooms divided into three rows, each of which has a masonry lined with opus signinum or cocciopesto, which, as Vitruvius writes, was mainly used in the construction of cisterns, aqueducts, thermal pools because it is suitable for waterproofing lime mortar. It is very evocative to come across obsolete inscriptions such as Calm, exit, dating back to the Second World War, when the premises were used as a shelter against bombings.