On the eastern edge of the Mojave Desert, an hour's drive southwest of Las Vegas in Mountain Pass, California, lies a 1.4-billion-year-old deposit of cerium, neodym-ium, and other metals that is the richest source of rare-earth elements in the United States. Beside hills populated by cacti, Joshua trees, and wandering tortoises is a vast waste dump of tan and white rocks that was built up over more than 50 years of production at a 50-acre open-pit mine here. The mine was once the world's biggest producer of these metals, which are crucial to such diverse products as computer hard drives, compact fluorescent light bulbs, and the magnets used in electric vehicles' motors. And the site still holds enough of them to mine for at least another 30 years. But in 2002 it was shut down, owing to severe environmental problems and the emergence of Chinese producers that supplied the metals at lower cost. The mine sat idle for a decade.
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