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Dredging worker Maihemuti Saibuduli goes down a karez to repair it in Turpan, China, in 2015.Zhao Ge

Tahir Tomur steps out of his car and looks in astonishment at the side of a dusty street, where a hole the size of a large sewer grate has been piled full of dirt and refuse.

"Oh my God, they've covered it," he says. "This is a karez!"

A few dozen metres away, he finds another similar hole stuffed with plastic bags and bits of wood.

"It's dried up and people have put garbage in it," Mr. Tomur says. "It's very bad."

He is walking through a neighbourhood in Turpan, an ancient oasis city that has flourished in the white-hot heat of an arid basin in China's western Xinjiang region, in part because of the history behind these holes.

These were once vertical shafts, plunging deep into the earth, where they connect with long horizontal tunnels. The karez were built to carry water from nearby mountain areas to the dry flatlands, whisking it below the ground where it is shielded from the evaporating heat that can approach 50 C in summer.

Dug by hand by the region's largely Muslim Uyghur people over many centuries, some 4,400 kilometres of those tunnels in this area once formed a feat of engineering with few parallels inside the borders of modern China. Dubbed the "underground Great Wall" by scholars, the structure was once used to irrigate vineyards and slake thirst – and China has, in recent years, sought to protect and restore what remains of it.

But in the past half-century, most of the karez tunnels have gone dry. A 2009 national survey found that only a quarter of Turpan's karez system still carried water. What was once a source of life and a wellspring of culture has been reduced to a parched historical artifact in a region where the position of the Uyghurs, too, has been diminished by heavy Han Chinese migration.

In Communist China, "no one cares about the karez anymore. No one looks after them," says Gheni, a Uyghur man walking past the hole stuffed with garbage.

"It's a pity, but what can we do?"

For Uyghurs, the drying of the tunnels has fed deeper worries; the crumbling fate of the historical waterways is linked with that of a people who themselves feel under assault. Uyghurs have been involved in a string of terror attacks in recent years, and fears of radicalization have prompted harsh crackdowns by Chinese authorities, who have restricted religious practice, installed pervasive surveillance systems and sent thousands to political re-education camps.

The karez are "a rare example of Uyghur ingenuity in a place where Uyghur aren't granted much to be proud of," says one Xinjiang resident, who asked not to be named for fear of reprisal.

Even speaking out about the state of the system today can be dangerous, Gheni says. "We cannot talk about this as Uyghurs," he explains. "Uyghurs cannot speak too much, or we will be taken away."

In general, China's record on cultural preservation has been spotty. Vast numbers of religious and historical relics were destroyed during the tumult of the Cultural Revolution; some of the country's most impressive imperial artifacts remain in places such as Taiwan.

Even the Great Wall, one of the country's most celebrated landmarks, has been devastated – the victim of neglect, weather and local villagers plundering bricks for their own use. Nearly 30 per cent of the Ming-era dynasty structure has vanished, state media have reported, and much of the remainder is in rough repair.

And while the karez system is less known than the Great Wall, it is arguably more important. The wall's defensive value has largely faded, but karez networks remain valuable sources of irrigation, drinking water and other daily needs; without them, vineyards and homes would be parched. Some villagers still maintain beds along the banks of the below-ground aqueducts, where summertime temperatures can be 20 degrees cooler.

But the condition of China's karez system compares poorly with similar installations elsewhere. Turpan's underground channels are similar in design and function to the extensive network of qanats that have for millennia irrigated arid areas in Iran, Syria and northern Africa (the technology, spread by the Spanish, reached as far as the site of modern-day Los Angeles). In Iran, roughly three-quarters of the qanats remain functional.

Turpan's karez, by comparison, have suffered from modernization, including state-led development initiatives that have intensified agricultural use of the land to raise cotton, and brought new technologies to secure sufficient water. The installation of electric pumps, in particular, has lowered water tables to the point where it no longer naturally flows down many tunnels. Pipes that feed kitchen taps and irrigation systems have also diminished the need for the ancient aqueducts.

Chinese authorities have nonetheless sought to halt karez deterioration. Since 2009, local officials in Turpan have spent more than $11-million (Canadian) on protection and restoration work. The goal is to keep 200 to 300 karez functional – about a quarter of what once existed – said a Chinese professor who has researched the network in detail, but also asked that his name not be used.

"All of the karez's that have survived have been revived," he says. He considers that a success: "If you look at karez's in Turkey, almost none can be used any more."

But maintaining the system is tough, the professor said. Dug out of earth, a karez has traditionally required annual maintenance to clear out mud and shore up collapsed earth. Today, though, it's hard to find someone willing to do that work. "Younger people don't want to go down, considering it is only a bit over a metre high" inside the tunnels, he says.

Modern repair methods have been more durable, including installation of concrete culverts and supports. Still, only some areas are worth fixing, the professor says. "Preserving the karez is an act of cultural heritage preservation," and in that, "there are certain rules. Only the best ones will be preserved."

But the tunnels have value that goes beyond history or even culture, argues Shalamu Abudu, a Uyghur scholar who is a postdoctoral research associate at Texas A&M. Ancient though they might be, they are a technology that remains valuable today for ecological reasons.

Where water wells can be drilled deep enough to suck dry underground aquifers, the karez system "is restricted naturally," he said. Only if sufficient water exists will it flow. "So from the sustainability point of view, I believe it's better."

That means, however, that any restoration of the karez will require an effort that goes beyond installing concrete. What's needed, Mr. Abudu says, is a systemic approach, one "that tries to preserve the groundwater."

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