Deep-sea mining is the idea of harvesting valuable rocks and metal-rich crusts that lie thousands of meters below the ocean’s surface.

On the flat, muddy plains of the Pacific and Indian Oceans, potato-sized “polymetallic nodules” sit loose on the seabed. Each nodule is like a mini battery pack, containing nickel, cobalt, copper and manganese — the same metals used in electric-car batteries, wind turbines and smartphone circuits.

Farther upslope, on dormant underwater volcanoes, wave after wave of mineral-laden seawater has coated the hard rock with cobalt-rich layers. And around hot hydrothermal vents, black-smoker chimneys build mounds of copper, zinc, gold and silver called seafloor massive sulphides.

Because these deposits lie 3,000 - 6,000 meters deep, no human dives to collect them. Engineers instead send down robotic crawlers that roll across the seabed, vacuum the nodules or chip off crusts, and pump the material up a long riser pipe to a ship. The ship separates the rocks from water and stores them for transport to shore-side refineries.

Unlike land mining, there is no need for open pits or mountain blasting, but the work must tackle other challenges: crushing pressure, near-freezing temperatures, and delicate deep-sea ecosystems.

Most of the ocean floor is beyond any single nation’s borders, so a United-Nations-created body — the International Seabed Authority — issues exploration licences and is writing detailed rules to balance mining with environmental protection.

Supporters say deep-sea metals could ease shortages for clean-energy technology and reduce reliance on politically sensitive land mines; critics warn that disturbing the seafloor could harm habitats.

As a result, deep-sea mining remains at the pilot stage, and every test run is watched closely by governments, researchers and companies hoping to unlock the planet’s last great resource frontier.