Polymetallic Nodules on the Deep Seafloor

Published June 14, 2026

The Deep Harvest, Chapter 2: The Ocean Floor

In a 2022 pilot run by NORI and Allseas, nodules came up from the seabed to a ship through a riser pipe about 4.3 kilometers long. The system held a sustained rate of 86.4 tonnes an hour. About 4,500 tonnes were recovered over the trial, and more than 3,000 tonnes of that made the full trip up the riser.

Nodules are lumps of metal-bearing rock lying loose on the deep ocean floor. A mine on land has to strip and haul away waste rock before it reaches ore. On the seabed the ore is already sitting on top of the sediment, and the collector's job is to pick it up.

The largest field of them is in the region known as the CCZ, 4,000 to 6,000 meters down. It holds about 21 billion tonnes of nodules, with average grades of about 1.3 percent nickel, 0.2 percent cobalt, 1.1 percent copper, and roughly 28 to 29 percent manganese. Other basins add to the count: about 3 billion tonnes in the Peru Basin, about 8 billion tonnes in the Central Indian Ocean Basin, and a cobalt-rich field in the Penrhyn Basin off the Cook Islands.

The contractor GSR ran its own trial with a collector called Patania II. Over 41 hours it gathered 660 tonnes, at 27.7 tonnes an hour. After the NORI and GSR runs, the question about collection has shifted from whether a machine can gather nodules at this depth to how well, and for how long, it can keep gathering them. Long-run uptime and operating cost after many quarters of steady work are still unknown, because the pilots were short trials.

The ship and the riser are process equipment, and they come with their own bill. Nodules come up wet, so offshore costs are quoted per wet tonne, the weight of the material as it is lifted, water included. TMC, a developer, claims offshore operating costs of $20.3 per wet tonne. A critique from Iceberg disputes that figure, and the research behind The Deep Harvest puts a realistic total offshore cost at about $40 to $50 per wet tonne. A commercial-scale operation is expected to cost $1 billion to $3 billion to build, with feasibility work pointing toward operating costs of about $200 per dry tonne once the water is removed.

The two sides disagree on the whole project, too. TMC's prefeasibility study shows a net present value, the project's lifetime earnings discounted back to today, of $18.1 billion and an internal rate of return of 35.6 percent. Iceberg recalculated the same project and got a net present value of about negative $721 million. The gap comes from disputes over how much metal the process will recover and what it will cost. The research behind the book records both numbers and does not settle the argument.

On shore, one route to the nickel and cobalt is high-pressure acid leaching, or HPAL: the ore goes into a pressure vessel with hot sulfuric acid, which dissolves the metals out. On land, HPAL plants typically run at 240 to 270 degrees Celsius and 33 to 35 atmospheres. They consume 350 to 500 kilograms of sulfuric acid for every tonne of ore. That acid is made from sulfur. The world produced about 85 million tonnes of sulfur in 2024, and about 90 percent of it was turned into sulfuric acid. Most of it comes from the Claus process, which strips sulfur out of oil and natural gas during refining. A nodule refinery running on HPAL depends on a supply of sulfur that comes from fossil fuel processing, and that dependence sets a limit on how many tonnes the refinery can push through.

HPAL is not the only flowsheet, the sequence of steps that turns ore into metal. The rotary kiln electric furnace route, RKEF, adds about $76 to $100 or more per wet tonne in processing cost. Cuprion is a closed-loop process that works at ambient temperature and recycles its reagents. Which of these wins at scale is still open.

The deep seabed offers one condition HPAL needs. Water in the CCZ sits at about 0.8 to 2.0 degrees Celsius under 400 to 600 atmospheres of pressure, far more pressure than an HPAL vessel uses on land, but none of the heat. The Deep Harvest raises an idea it calls the Abyssal Autoclave, an autoclave being a sealed vessel that cooks material under pressure, to leach ore at depth and use the ocean's pressure for free. It is speculative. Nobody has shown it works.

Nodules are also only one of three seabed deposits, and each needs its own machine. Seafloor massive sulfides are solid deposits of metal-bearing sulfide rock. At Solwara 1 they graded about 6.8 percent copper and 4.8 parts per million gold. Nautilus, the company that tried to mine Solwara 1, failed for financial, regulatory, and reputational reasons, not because the engineering could not be done. Cobalt-rich crusts are a third kind, coating rock at about 800 to 2,500 meters, with cobalt up to about 1.7 percent and commercial averages of 0.3 to 1.0 percent. They grow by millimeters per million years.

The collector also leaves something behind. Monitoring of the NORI trial with DHI found sediment plumes, the clouds of mud stirred up by the collector, stayed more localized than earlier models predicted. Recovery of the seabed itself is slower. Earlier study sites such as DISCOL point to recovery timescales of decades at minimum, and how the seabed recovers beyond the windows that have been watched so far is not known.

The International Seabed Authority, or ISA, has issued 19 exploration contracts, and contractors have spent large sums of private money under them. The rules for actual mining, the Mining Code, were still being written when the research was done, and the royalty a miner would pay is not settled. Every cost above, from the $1 billion to $3 billion build to the disputed net present value, assumes terms the unfinished code has not yet set. A company can prove its collector and choose its refinery and still have no approved mining rules to plan a commercial operation around, so the next tonne of seabed nickel waits on the ISA and on whoever will fund a project before the code is done.

The Deep Harvest works through the collector, the acid, and the code in Chapter 2, The Ocean Floor.