The deep ocean was once assumed to be lifeless and barren. Today we know that even the deepest waters teem with living creatures, some of them thought to be little changed from when life itself first appeared on the planet. The deep ocean is also essential to the earth’s biosphere—it regulates global temperatures, stores carbon, provides habitat for countless species, and cycles nutrients for marine food webs. Currently stressed by pollution, industrial fishing, and oil and gas development, these cold, dark waters now face another challenge: mining. With land-based mineral sources in decline, seabeds offer a new and largely untapped frontier for mineral extraction, and companies are gearing up to mine a treasure trove of copper, zinc, gold, manganese, and other minerals from the ocean floor.