La Malga
About
At least two dozen vaulted cisterns once stood in a line at La Malga, each roughly 130 Roman paces long, storing tens of thousands of cubic meters piped in from the Zaghouan aqueduct to supply Carthage and its baths. Hadrian ordered that aqueduct built starting around 128 CE, so these cisterns could not yet have held water at this map's 117 CE snapshot. Carthage instead relied on wells and smaller cisterns at Trajan's death. The ruins, later stables, are now overgrown with fig trees.
What happened here
Underground, vaulted, and much larger inside than anything visible above suggests. The walls are lined with waterproof mortar, the corners rounded so nothing can lodge, and the roof carried on rows of piers. Water arrives by aqueduct or off the roofs and is held against the dry months, for a fleet, a bath house, a fort or a whole town. Sediment settles in a trap before the main tank, and the tank is drained and scrubbed on a cycle by men who go down by ladder. Access is through shafts with stairs, and it is cool enough down there that things are stored in them. A town on a dry site lives or dies by its cisterns, and they are among the first things built.
Sources
Roman Maps shows the empire frozen at 11 August 117 CE, the day Trajan died — its moment of greatest territorial extent. Explore the full map →