Cut deep into the desert rock of southern California are the jagged tiers of an open-pit mine. Mountain Pass is North America's only mine of rare-earth metals, used in everything from fighter jets to the drive-trains of electric cars. In 2015 Mountain Pass shut, unable to compete with rare-earth producers in China. But it has begun a new chapter. mp Materials, which bought the mine in 2017, said on March 18th that production in 2020 jumped by 40%. More expansion is planned. With grants awarded by America's defence department last year, mp Materials will build facilities to process rare earths, part of an effort to secure supply independent from China.