An engineering solution to extreme heat
The Turfan Depression is the second-lowest exposed point on Earth and one of its hottest and driest. Summer surface temperatures in the surrounding desert can reach 80°C, and annual rainfall is a scant 16.4 mm. To survive, ancient inhabitants developed the karez system, an irrigation network that brings water underground from the Tian Shan mountains. By keeping the water in subterranean channels, it avoids the extreme evaporation that would otherwise consume it.
The system's design is a precise application of gravitational physics. It consists of four main parts: vertical wells, underground canals, surface canals, and small reservoirs. Workers began by digging a "mother well" deep into the water table at the base of the mountains—some shafts are over 100 meters deep. From the bottom of this well, they dug a gently sloping horizontal tunnel toward the oasis. To excavate, they would dig vertical shafts every 20 to 70 meters along the planned route, removing soil and rock by hand with tools like karez picks and baskets. These shafts also provide ventilation. The underground tunnels, typically about 2 meters high, connect the bottoms of the vertical wells. The gentle gradient allows water to flow steadily without eroding the channel walls. When the tunnel finally emerges at the lower-elevation oasis, the water flows into surface canals and reservoirs for irrigation.
A declining ancient water system
The oldest karez systems date to the Han Dynasty, over 2,000 years ago. The technology likely arrived from Persia via the Silk Road, where similar systems called qanats were already in use. This network turned the barren Turfan Depression into an important oasis, supporting agriculture and cities along the trade route. At its peak, more than 1,700 individual karez lines existed, with a combined length of over 5,000 kilometers. This massive scale is called "the underground Great Wall".
Today, the system is under threat. While over 1,100 karez channels remain, many are dry or in disrepair. The primary cause is the widespread use of modern tube wells powered by electricity. These wells draw from the same underground aquifers but pull water from much deeper levels, lowering the water table below the intake of the gravity-fed karez. As of the early 2020s, only about 300 to 400 karez are still active, supplying around 16% of the region's irrigation water.