Roman cisterns

A cistern is a store of water, and most Roman ones are plain: a tank, often underground, that caught rain or held water brought by an aqueduct. The Piscina Mirabilis at Bacoli shows how large one could be. It is an underground hall of vaulted aisles on 48 pillars, about 70 m long, which held an estimated 12,600 m³ of water and is counted among the largest cisterns the Romans built (Pleiades; en.wikipedia). Pleiades suggests it supplied the sailors of the Roman fleet with drinking water.

A cistern is worth visiting for the inside, not the outside. From the street the Piscina is a long wall with buttresses; the reason to come is the interior, which is entered accompanied, on a timetable, with a helmet (the ticket office).

Inside, the design is practical. Light and air entered through windows along the top of the side walls and through openings in the roof; the walls and pillars were lined with waterproof plaster, and a settling basin in the central aisle was used to clean and drain the cistern (it.wikipedia; en.wikipedia). Water came to the Piscina by the Aqua Augusta, the aqueduct that served at least eight towns around the Bay of Naples (en.wikipedia).