Picture a rock the size of a potato that took far longer to form than our species has existed. That is roughly what a polymetallic nodule is. These metal-rich lumps sit on the deep-sea floor and build up one layer at a time, at rates so slow that a whole nodule may take millions to tens of millions of years to form.

That number reframes the argument about deep-sea mining before you even reach the ecology. You are not scooping up ordinary gravel. You are scooping up geological time.

A note before going further: we are not marine scientists, geologists, or policy experts. This is reading and reflection on published research, much of it drawn from a small number of test sites, not a verdict on whether seabed mining should happen.

What the nodules actually are

Polymetallic nodules, sometimes called manganese nodules, are concretions rich in manganese and iron that lie on or partly within the muddy abyssal seafloor, commonly at depths of 4,000 to 6,000 metres. Each grows around a small nucleus, such as a shark tooth, shell fragment, grain of sand or piece of rock, slowly wrapping itself in layers of iron and manganese oxides. The best-known deposits lie in the Pacific’s Clarion-Clipperton Zone, between Hawaii and Mexico.

What makes them commercially interesting is what those layers contain: nickel, cobalt, copper and manganese, along with smaller amounts of other elements. These metals are used across batteries, electricity networks and other energy technologies, as well as steelmaking and manufacturing. Because nodules rest at the sediment surface rather than inside solid rock, mining companies argue they could be collected without digging conventional pits or blasting mountains.

The growth rate that makes geologists blink

Scientific American described one specimen as growing at 1 to 10 centimetres every million years and estimated that it had taken about 10 million years to form.

A French oceanographic institute says visible nodules in the areas it describes are generally 10 to 15 million years old at most. The safest summary is that centimetre-scale nodules form over millions, and often tens of millions, of years.

Whatever comes up the pipe cannot be replaced on any timescale meaningful to us. Not in a lifetime, and not in a civilisation. The nodules are effectively non-renewable.

Why mining firms want them anyway

The commercial logic is straightforward. Michael Clarke, an environmental manager at The Metals Company, described the proposed method to NPR: the machine “sucks these nodules up and sends them up a pipe vertically to a vessel that’s on the surface.” 

The Metals Company is pursuing United States authorisations for exploration and commercial recovery. The company says its first planned commercial system would have a capacity of about three million wet tonnes of nodules a year, subject to regulatory approval. The International Seabed Authority has still not adopted a final exploitation code for mining in international waters, while the United States is pursuing a separate process under its own law.

What we stand to lose, and what we don’t yet know

The nodules are not just ore. They are also hard surfaces used by sponges, corals and other organisms on an otherwise soft plain. Remove them and nodule-dependent habitat disappears.

MBARI senior scientist Steve Haddock called it “a bit of a fallacy that there’s not much down there, it’s not very important and our impact is going to be restricted to a small area.” The industry argues that seabed collection could still have lower impacts than some land-based mining.

The evidence comes from a limited number of tests. Two months after one industrial-scale trial, researchers found that animal density inside the mining tracks had fallen by 37 percent and species richness by 32 percent.

Sabine Gollner of the Royal Netherlands Institute for Sea Research said: “it’s very clear that if you take the nodules away, the ecosystem would move to a different state for millions of years.”

Antje Boetius, MBARI’s president and chief executive, put the unknowns more simply: “We don’t know what the value of all of those lifeforms and their genetic resources are for future generations of humans.”

So the nodule ends up being two things at once: a store of metals sought by modern industry, and the foundation of an ecosystem we have barely begun to describe. Taking the first probably means permanently removing the second from the mined area, long before we fully understand what was there.