Big companies have looked into the idea of mining the ocean floor since the 1960s and 1970s. They proved the principle by collecting hundreds of tonnes of manganese nodules—potato-sized mineral. At first sight, these nodules are attractive targets for mining because, besides manganese, they are rich in cobalt, copper and nickel. As a commercial proposition, though, the idea never caught on. Working underwater proved too expensive and prospectors discovered new mines on dry land. Worries about shortages went away, and ocean mining returned whence it had come, to the pages of science-fiction novels. But now it is coming back.