The 9C Aghlabid Basin is a huge 5m-deep pool with 64 sides. Built to provide the city with water, this 55m³ basin was supplied by a 36km aqueduct which carried water down from the mountains. Once home to 250 000 inhabitants (the population today is 140 000), the city originally had 14 of these reservoirs. These basins were always accompanied by a smaller pool, which was used for sedimentation. A second complex of basins, about 50m to the northeast of the tourist office, has just been restored.