The Potato-Sized Rocks That Could Decide the Clean Energy Transition

Polymetallic nodules are the most commercially targeted resource in deep-sea mining — and understanding them is essential to understanding why the race to mine the ocean floor has accelerated so dramatically in the past decade. These lumpy, potato-sized rocks have been sitting on the seafloor for tens of millions of years. Now they sit at the centre of a multi-trillion-dollar resource race, a governance crisis at the United Nations, and one of the most contested environmental debates of the clean energy transition.

What Are Polymetallic Nodules?

Polymetallic nodules (also called manganese nodules or ferromanganese nodules) are mineral accretions that form on the deep seafloor, typically at depths of 4,000–6,000 metres. They range from a few millimetres to over 20 centimetres in diameter, though most commercial deposits consist of nodules in the 2–8 cm range.

They form through two processes:

Hydrogenous precipitation: Metals dissolved in seawater — principally manganese, iron, cobalt, and nickel — slowly precipitate onto a nucleus over millions of years. The nucleus can be a shark tooth, a fragment of volcanic rock, or even a fragment of an older nodule.

Diagenetic precipitation: Metals flux upward from the seafloor sediment through biological and chemical processes, also accreting onto the nodule surface. This process tends to concentrate nickel and copper.

Most commercially interesting nodules involve both processes.

Why Are They Worth Billions?

The value of polymetallic nodules is a function of both their metal content and the metals they contain.

Nickel and cobalt are battery metals. Lithium-ion batteries in electric vehicles, grid storage systems, and consumer electronics rely on nickel-manganese-cobalt (NMC) cathode chemistries. Both nickel and cobalt supply chains are dominated by a small number of countries — Indonesia and the Philippines for nickel, the Democratic Republic of Congo for cobalt — creating supply-chain risks that Western governments and manufacturers are anxious to diversify away from.

The scale is extraordinary. The Clarion-Clipperton Zone (CCZ) alone is estimated to contain 21 billion tonnes of nodules. Even at conservative nickel grades, this represents more nickel than all known terrestrial reserves combined. The United States Geological Survey and independent researchers estimate the CCZ nodules contain:

• 340 million tonnes of nickel
• 290 million tonnes of copper
• 44 million tonnes of cobalt
• 7.1 billion tonnes of manganese

At 2024 commodity prices, the contained metal value of CCZ nodules exceeds $3 trillion — though this figure is theoretical, not recoverable, given the costs, engineering challenges, and regulatory uncertainty involved in deep-sea mining.

How Do They Form — and Why Does It Matter?

Nodule growth rates are measured in millimetres per million years. A nodule 5 cm in diameter may be 5–10 million years old. This extraordinarily slow formation rate is central to the environmental debate around mining:

Nodules are non-renewable on any human timescale. Unlike terrestrial mineral deposits, which can theoretically regenerate through geological processes on timescales of thousands of years, a harvested nodule field will not recover for millions of years.

They are habitat, not just rock. The nodule surface is the only hard substrate in the surrounding soft-sediment abyssal plain. Sponges, xenophyophores (the world's largest single-celled organisms), polychaete worms, brittle stars, and sea cucumbers all colonise nodule surfaces.

Where Are the Largest Deposits?

Nodule deposits are found across the Pacific, Indian, and Atlantic Oceans, but commercial interest is concentrated in a handful of regions:

Clarion-Clipperton Zone (Pacific): The most heavily contracted region. 17 ISA exploration contracts. Estimated 21 billion tonnes of nodules.

Peru Basin (Pacific): A major research focus. The DISCOL disturbance experiment in 1989 is still monitored here, providing the longest-running dataset on seafloor recovery from mining-scale disturbance.

Central Indian Ocean Basin: Home to ISA contracts held by India and others. Less studied than the CCZ.

The Environmental Science Debate

Deep-sea mining proponents frequently argue that nodule mining has a lower environmental footprint than equivalent terrestrial mining. Critics argue this comparison is misleading:

• Terrestrial mining impacts are known and partially managed. Deep-sea ecosystem responses to industrial disturbance are not yet understood well enough to predict, let alone mitigate.
• Recovery times are incomparable. Terrestrial ecosystems disturbed by mining can recover in decades to centuries. Deep-sea abyssal communities disturbed by nodule harvesting are not expected to recover in millions of years.
• Sediment plume impacts are transboundary. A plume generated in a contractor's licensed area does not respect contract boundaries.

Tracking Nodule Mining Concessions

Abyssal Claims maps every active ISA exploration contract — including all nodule contracts — overlaid on hydrothermal vent fields, OBIS biodiversity data, and APEI boundaries.