The Dujiangyan irrigation system has been a focal point in the debate over the mainland's hydropower development, with several controversial dams built around it and others planned. Built in about 250BC during the Qin dynasty, it is the world's oldest flood control and irrigation system still in use, according to Unesco when making the project a world cultural heritage site in 2000. Dujiangyan controls the flood-prone Min River and distributes it throughout the fertile Chengdu plains. Situated 55km northwestern of Chengdu , at the junction of the Qinghai-Tibet plateau and the Sichuan Basin, the waterworks irrigate about 733,000 hectares of farmland and provide water to Chengdu and six other cities, covering 27.5 per cent of Sichuan's population. Although experts have questioned Unesco's conclusion that the water facilities function 'perfectly', it still attracts many visitors.