The ancient Karez system is comprised of a series of wells and linking underground channels that uses gravity to bring ground water to the surface, usually far from the source. It is considered as one of the three great projects in China with the other two being the Great Wall and the Grand Canal. Originally ancient oasis towns depend on the streams and rivers nearby into which glaciers in far-off mountains feed. As the glaciers gradually shrank over the centuries, the streams they fed likewise diminished, resulting in less or water flowing to the oasis towns. Then people ingeniously created the karez to draw the underground water to irrigate the farmland.
Wells begin at the base of the mountains along the contours of the hillside.
To keep the underground channels unclogged, two men and a draught animal work as a team - one man is lowed down to clear the tunnel and buckets of mud are hoisted to the surface by the animal. The tunnels slope less than the contours of the geographical depression, so that the water reaches the oasis close to ground level. The water in karez will not evaporate in large quantities even under the scorching heat and fierce wind, hence ensuring a stable water flow and gravity irrigation. The history of Karez can be traced back to the Han dynasty. Most of today's karezes in the Turpan were built in the Qing dynasty and in after years.