Almost a league beneath the Pacific Ocean lies a treasure trove: 270m tonnes of nickel and 44m tonnes of cobalt. It got there particle by particle over millions of years, drifting down to form metallic lumps called nodules. These sit in a 4.5m-square-kilometre patch of seabed called the Clarion-Clipperton Zone (CCZ), 800km south-east of Hawaii, and can be collected using a robot that is part combine harvester and part vacuum cleaner. The metals could help meet long-term demand during the energy transition from fossil fuels, while reducing the human suffering and ecological damage that accompanies the mining of cobalt and nickel on land.