The Cisterns of La Malga, on the western edge of ancient Carthage, are the largest Roman hydraulic complex surviving in Africa — a vast set of parallel barrel-vaulted chambers built to store water delivered from the mountains by the Zaghouan aqueduct. At their peak they held something like 50,000 cubic metres, supplying both the public baths and the wealthier private houses of the imperial city.
The setting is unprepossessing — the cisterns sit on the edge of an outlying suburban neighbourhood, mostly unvisited and unprotected — but the scale of the structures is genuinely impressive. The vaults of the largest chambers are still intact and some of the smaller cisterns continue to hold water and serve local farmers. Bring a torch and watch your footing. The complex is rarely supervised and there is no entry fee.