The 9C Aghlabid Basin is a huge 5m-deep pool with 64 sides. Built to provide the city with water, this 55 000m3 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.