From the air it looks like a huge lake, fed by many tributaries, but on the ground it turns out to be a murky expanse of water, in which no fish or algae can survive. The shore is coated with a black crust, so thick you can walk on it. Into this huge, 10 sq km tailings pond nearby factories discharge water loaded with chemicals used to process the 17 most sought after minerals in the world, collectively known as rare earths.

The town of Baotou, in Inner Mongolia, is the largest Chinese source of these strategic elements, essential to advanced technology, from smartphones to GPS receivers, but also to wind farms and, above all, electric cars. The minerals are mined at Bayan Obo, 120km farther north, then brought to Baotou for processing.

The concentration of rare earths in the ore is very low, so they must be separated and purified, using hydro-metallurgical techniques and acid baths. China accounts for 97% of global output of these precious substances, with two-thirds produced in Baotou.

Quick Guide What are rare-earth metals and what are they used for? Show What are rare-earth metals? Rare earths refer to a group of 17 elements that are prized for their unique magnetic and electrochemical properties. They include elements such as cobalt, gadolinium, lanthanum, cerium and promethium, and are vital in the production of cancer treatment drugs, smartphones and renewable energy technologies. Where do they come from? They are not actually very rare and can be found across the Earth’s crust. However, they are often in low concentrations and are difficult and expensive to mine. The process can also damage the environment, with ecosystems put at risk by pit mining, the release of metal byproducts from refineries, and water contamination from particles being dumped during waste disposal. What are they used for? The use of rare earths is vast and varied, spanning national defence to consumer goods. They are used in healthcare, including for surgical supplies, pacemakers, cancer treatment drugs and rheumatoid arthritis medication. They are also found in telescope lenses, aircraft engines and are used as catalysts in auto exhaust systems to help reduce emissions. Neodymium is often used to produce high-powered, infrared lasers for national defence, while military suppliers such as BAE systems use rare earths to produce sensors for missile systems. Kalyeena Makortoff

The foul waters of the tailings pond contain all sorts of toxic chemicals, but also radioactive elements such as thorium which, if ingested, cause cancers of the pancreas and lungs, and leukaemia. “Before the factories were built, there were just fields here as far as the eye can see. In the place of this radioactive sludge, there were watermelons, aubergines and tomatoes,” says Li Guirong with a sigh.

It was in 1958 – when he was 10 – that a state-owned concern, the Baotou Iron and Steel company (Baogang), started producing rare-earth minerals. The lake appeared at that time. “To begin with we didn’t notice the pollution it was causing. How could we have known?” As secretary general of the local branch of the Communist party, he is one of the few residents who dares to speak out.

Towards the end of the 1980s, Li explains, crops in nearby villages started to fail: “Plants grew badly. They would flower all right, but sometimes there was no fruit or they were small or smelt awful.” Ten years later the villagers had to accept that vegetables simply would not grow any longer. In the village of Xinguang Sancun – much as in all those near the Baotou factories – farmers let some fields run wild and stopped planting anything but wheat and corn.

A study by the municipal environmental protection agency showed that rare-earth minerals were the source of their problems. The minerals themselves caused pollution, but also the dozens of new factories that had sprung up around the processing facilities and a fossil-fuel power station feeding Baotou’s new industrial fabric. Residents of what was now known as the “rare-earth capital of the world” were inhaling solvent vapour, particularly sulphuric acid, as well as coal dust, clearly visible in the air between houses.

Now the soil and groundwater are saturated with toxic substances. Five years ago Li had to get rid of his sick pigs, the last survivors of a collection of cows, horses, chickens and goats, killed off by the toxins.

The farmers have moved away. Most of the small brick houses in Xinguang Sancun, huddling close to one another, are going to rack and ruin. In just 10 years the population has dropped from 2,000 to 300 people.

Lu Yongqing, 56, was one of the first to go. “I couldn’t feed my family any longer,” he says. He tried his luck at Baotou, working as a mason, then carrying bricks in a factory, finally resorting to selling vegetables at local markets, with odd jobs on the side. Registered as farmers in their identity papers, the refugees from Xinguang Sancun are treated as second-class citizens and mercilessly exploited.

The farmers who have stayed on tend to gather near the mahjong hall. “I have aching legs, like many of the villagers. There’s a lot of diabetes, osteoporosis and chest problems. All the families are affected by illness,” says He Guixiang, 60. “I’ve been knocking on government doors for nearly 20 years,” she says. “To begin with I’d go every day, except Sundays.”

By maintaining the pressure, the villagers have obtained the promise of financial compensation, as yet only partly fulfilled. There has been talk of new housing, too. Neatly arranged tower blocks have gone up a few kilometres west of their homes. They were funded by compensation paid by Baogang to the local government.

But the buildings stand empty. The government is demanding that the villagers buy the right to occupy their flat, but they will not be able to pass it on to their children.

Some tried to sell waste from the pond, which still has a high rare-earth content, to reprocessing plants. The sludge fetched about $300 a tonne.

But the central government has recently deprived them of even this resource. One of their number is on trial and may incur a 10-year prison sentence.

This article originally appeared in Le Monde