Piscina Mirabilis is the largest and most monumental cistern of drinkable water ever built by the ancient Romans. It had the function of supplying water to the numerous ships belonging to the Classis Misenensis of the Roman Navy, once moored in the nearby port of Miseno. The cistern, also known as "the Water Cathedral " for its largeness and majesty, is located in Bauli, the name that the ancient Romans gave to the current city of Bacoli.

Piscina Mirabilis, partly dug into the tuff, had the capacity of 12,600 cubic meters of water! It was built on a quadrangular plan to obtain four rows of twelve cruciform pillars that divide the interior space into five long and thirteen short naves to support the vault. On the latter it is set the roof terrace paved in terracotta fragments and lime, communicating with the interior by several doors.

Piscina Mirabilis represented the landing and arrival point in Bacoli of the ancient Augustan aqueduct which, from the spring of Serino (AV), located at an altitude of 330 meters and with a journey of 100 kilometers about, brought water to Naples and Campi Flegrei up to the eight meters of Piscina Mirabilis (now at an altitude of 2, due to bradyseism). The Augustan aqueduct was destroyed, between the 4th and 5th century AD. C and Piscina Mirabilis has no longer been use and as a matter of fact it is now emptied of water.