The metals lying on the abyssal plain in the form of polymetallic nodules include nickel, cobalt and manganese in concentrations that would be attractive on land, and collecting them requires no blasting or excavation, only lifting them from the sediment. This is the case its proponents make: a supply of battery metals obtained without the deforestation, tailings dams and labour conditions associated with terrestrial mining of the same materials. The comparison is fair as far as it goes, and it rests on an assumption that has not been tested, namely that the seabed damage is smaller than the damage avoided. Nobody currently knows the exchange rate between the two. Both sides of that ledger are being estimated by people with an interest in the answer.