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It's not a temple as such, but this semi-circular structure, built under Hadrian, is a monument to the mountain-spring source that kept Carthage watered. The ancient fountain is surrounded by arched niches that once held 12 statues depicting the months of the year. In the rocks behind the temple, the method of excavating these huge blocks can still be seen in some of the cracked stones. There's some on-site signage, though it's only in French and Arabic.
The Water Temple is about 2km southwest of Zaghouan's old town along the well-signposted continuation of Rue Sidi Ali Azouz, on a forested hillside.