The Piscina Mirabilis was a freshwater cistern on the Bacoli cliff at the western end of the Gulf of Naples, southern Italy. One of the largest freshwater cisterns built by the ancient Romans, it was situated there in order to provide the Roman western imperial fleet at Portus Julius with drinking water.
The cistern was dug entirely out of the tuff cliff face and was 15 metres high/deep (c. 49 feet), 72 metres long (c. 236 feet), and 25 metres wide (c. 82 feet). The capacity/volume was 12,600 cubic metres (c. 445,000 cubic feet) . It was supported by vaulted ceilings and 48 pillars. It was supplied with water from the main Roman aqueduct, the Aqua Augusta, that brought water from sources in Serino near Avellino, 100 kilometres distant, to Naples.