Chinese mines and refineries produce most of the world’s rare earth metals and practically all of a few crucial kinds of rare earths. This has given China’s government near complete control over a critical choke point in global trade.

But for decades in northern China, toxic sludge from rare earth processing has been dumped into a four-square-mile artificial lake. In south-central China, rare earth mines have poisoned dozens of once-green valleys and left hillsides stripped to barren red clay.

Achieving dominance in rare earths came with a heavy cost for China, which largely tolerated severe environmental damage for many years. The industrialized world, by contrast, had tighter regulations and stopped accepting even limited environmental harm from the industry as far back as the 1990s, when rare earth mines and processing centers closed elsewhere.

In China, the worst damage occurred in and around Baotou, a flat, industrial city of two million people in China’s Inner Mongolia, on the southern edge of the Gobi Desert. Baotou calls itself the world capital of the rare earth industry, but the city and its people bear the scars from decades of poorly regulated rare earths production.

An artificial lake of sludge known as the Weikuang Dam, four square miles in size, holds the waste left over after metals are extracted from mined ore. During the winter and spring, the sludge dries out. The dust that then blows off the lake is contaminated with lead, cadmium and other heavy metals, including traces of radioactive thorium, according to technical papers by Chinese scholars.

During the summer rainy season, the sludge becomes coated with a layer of water that mixes with poisons and thorium. This dangerous mix seeps into the groundwater underneath the lake.