Such projects plan to dig up rocks from the sea floor and transport them to ships at the surface using hydraulic pumps1. But deep-sea mining doesn't remove all environmental concerns from the inhabited land; once ground into slurry, the crushed mixture would be transported onshore for processing. Extracting the metals often requires large amounts of toxic chemicals, such as cyanide and mercury — a process with a poor track record on land. For example, in 2000, a cyanide spill from the Baia Mare gold mine in Romania contaminated the drinking water of 2.5 million people and was dubbed Europe's worst environmental disaster since Chernobyl. And small-scale, artisanal gold mining — a common and unregulated practice in cities such as La Rinconada, Peru — is now thought to be the leading source of mercury pollution globally2. As yet, it is unclear where the onshore processing of deep-sea minerals will take place, but it is likely that developing nations with lower safety restrictions will bear the brunt.