郑国渠

Named after its engineer, the Han-state water expert Zheng Guo 郑国. The state of Han sent him to Qin in 246 BC ostensibly as an agricultural advisor — but secretly to drain Qin's resources on a massive irrigation project and prevent it from invading Han. The plan backfired spectacularly: when discovered, Zheng Guo argued the canal would benefit Qin for generations, and the king of Qin (the future First Emperor) agreed and let him finish it. Within a decade Qin had used its new agricultural wealth to conquer all six rival states — including Han.

Together with the Dujiangyan 都江堰 in Sichuan and the Lingqu 灵渠 in Guangxi, considered one of ancient China's three great hydraulic engineering achievements. Parts of the original system, now expanded, still irrigate around 150,000 hectares of farmland in Shaanxi today.