The building overlooking the harbour of Misenum, built in the Augustan period to ensure the water supply for the military base and the city as a whole, was named Piscina mirabilis by the seventeenth-eighteenth-century antiquarian. The structure, also designed by Giuliano Sangallo because of his regal architecture, is a vast cistern with a capacity of 12,600 cubic metres. Excavated out of the tufa rock, it has a quadrangular plan and is divided internally into five long naves and thirteen smaller ones by four rows of twelve cruciform pillars supporting a barrel vault, above which rises a roof terrace covered in cocciopesto, connected to the interior by a series of doors. The outer walls are made of opus reticulatum, while the side walls are covered with brick, the pillars are made of tufa. On the inside, a 1.10 m deep basin was excavated, with an outlet at one end, which served as a decanting and draining basin to keep the cistern clean, fed by an inlet duct located near the western entrance. The windows also allowed for a fair amount of light and adequate ventilation. On the exterior, on the north-east side, twelve small rooms were also built, covered by barrel vaults, with a floor 1.80 metres lower than the vault of the cistern, and in opus mixtum and listatum, with a cocciopesto kerb at the bottom of the pillars. These spaces would have been an extension to the hydraulic system, built in the late 1st and early 2nd century AD.