Certainly it is the biggest Roman cistern ever known till now with a volumetric capacity of 12.600 m3 of water, to which it is possible to accede from Via S. Anna.
This building of Augustan Age, situated up the hill facing the sea, was the terminal point of the aqueduct of Serino.
The cistern is architecturally spectacular, dug in the tuff for 70 metres of length, 25.50 of width and 15 of depth. The building presents a rectangular plan with 48 pillars in reticulated work with components of tuff and divided into four lines which delimitate the inner room of the cistern.
The remarkable architectonic effect led antiquarians to attribute to the building the name “Piscina Mirabile” (Admirable Piscina).
The walls of reticulated work with lateritious components are covered with a thick layer of cocciopesto (mortar with potsherd); there is a basin of 1.10 metres, probably a polishing pool, that is a waste-bath for the maintenance of the cistern, in the floor of the nave.
Probably the water was drawn from the cistern by hydraulic machines and then it was canalized. This hypothesis seems to be confirmed by the lack of openings at the walls. There is a terrace of cocciopesto (mortar with potsherd) at the entrance, supported by the barrel covering of the cistern.