Seafloor food still hasn’t recovered 44 years after a deep-sea mining test

The tracks a nodule-mining machine cut into the floor of the Pacific Ocean more than four decades ago still hold less food than the seabed around them. Sediment inside the furrows carries less organic matter, and organic matter of poorer quality, than undisturbed ground a few metres away, and fewer animals live where that food is thinnest.

That matters because the same stretch of ocean, the Clarion-Clipperton Zone between Hawaii and Mexico, is where mining companies want to start collecting metal-rich nodules on an industrial scale. Most of what scientists know about how the abyss recovers comes from short experiments. This one has been running by accident for forty-four years, and the food supply that every animal down there depends on has still not come back.

The study, led by marine biologist Rachel Jeffreys of the University of Liverpool and published in the journal Elementa: Science of the Anthropocene, examined the site of a test mining run by the Ocean Minerals Company, known as OMCO. “Remarkably, we can still detect the effects of a mining test 44 years after it took place,” Jeffreys said.

On the abyssal plains, which lie between 3,000 and 6,000 metres down, food arrives as a slow drizzle of particles sinking from the sunlit surface: dead plankton, faecal pellets, fragments of everything that lived kilometres above. The paper calls this particulate organic matter the primary food source for seafloor animals and a key factor in how many species live there.

A mining machine on a spy ship

The test took place in 1979. The Hughes Glomar Explorer, a ship originally built for the CIA’s secret attempt to raise a sunken Soviet submarine, lowered a self-propelled collector to the seabed on a steel riser made of 18-metre pipe sections. The machine was 9 metres wide, 14 metres long and 4.5 metres high, weighed around 100 tonnes, and drove across the mud on Archimedes screws at a depth of roughly 4,700 metres.

Its job was to scoop up polymetallic nodules, potato-sized lumps of minerals that grow on the seafloor over millions of years. They contain metals used in rechargeable batteries, touch screens and other electronics, which is why the Clarion-Clipperton Zone, carpeted with them, attracts so much commercial interest.

When the collector left, it left behind two deep propulsion furrows and, between them, a strip of seabed cleared of nodules. A study of the same site published in Nature in 2025 found that the furrows, 0.2 to 0.8 metres deep and 1 to 3 metres wide, look almost exactly as they did in photographs taken right after the test.

How they did it

The fieldwork belonged to SMARTEX (Seabed Mining and Resilience To EXperimental Impact), a UK-funded project led by Daniel Jones of the National Oceanography Centre and Adrian Glover of the Natural History Museum in London. In 2023 the team spent a month aboard the research ship RRS James Cook above the old test site, using a remotely operated vehicle to survey the tracks and collect sediment samples at around 4,800 metres.

They sampled three kinds of ground: the collector tracks themselves, the plume area next to them where sediment stirred up by the machine had settled back down, and a control site that was never touched. In the companion Nature study, the plume samples came from about 10 metres from the track and the control area from roughly 2 kilometres away.

For each core, the team measured how much food the sediment held and how good it was. Total organic carbon and nitrogen give the quantity. Stable carbon and nitrogen isotopes, chemical fingerprints of where the material came from and how far it has decayed, speak to quality. So do lipid biomarkers, the fats in the sediment, including omega-3 fatty acids that deep-sea animals need to grow and reproduce. The researchers also measured the size of the sediment grains and compared all of it with counts of animals from still and video images of the same ground.

Counting animals alone would show that something is missing. Measuring the food shows a possible reason why.

What the tracks are missing

Inside the collector tracks, both the quantity and the quality of organic matter were lower than in the surrounding area, and more patchy from one sample to the next. The grain sizes in the tracks also differed from those outside them, and the organic-matter patterns followed those differences. The machine changed the sediment itself, and the altered sediment holds a poorer diet.

The animals followed the food. Densities of foraminifera (single-celled organisms with shells), xenophyophores (giant single-celled organisms that can grow larger than a coin) and megafauna, the animals big enough to see in photographs, were higher at the control site and in the plume area, where organic matter quality was highest. The plume area, which was only dusted with resettled sediment, looked much like the untouched seabed. The damage concentrated where the collector actually ran.

“Changes in the quantity and quality of organic matter could affect the food available to organisms living on the seafloor and, in turn, influence how ecosystems recover from disturbance,” Jeffreys said.

Why forty-four years is not long down there

The abyss runs on a different clock. Natural sedimentation in the Clarion-Clipperton Zone adds only 1.5 to 11 millimetres of new material every thousand years, according to the Nature study of the site. At the fastest of those rates, forty-four years deposits about half a millimetre, far too little to bury a scar or replace the layer a mining machine disturbed.

The 2025 Nature paper, led by Jones, counted some signs of recovery in the tracks. Sediment-dwelling worms, crustaceans and molluscs were back at slightly lower but broadly similar densities, mobile sea cucumbers gathered in the furrows, and new xenophyophores more than 50 millimetres across had settled at up to about eight per square metre. But animals that live fixed in place, such as sponges, remained scarce in the disturbed ground. The new Elementa paper adds the layer underneath those counts: the food supply in the tracks is still depleted, and the animal numbers track it.

What it doesn’t settle

The OMCO test was small. It disturbed a few narrow tracks, while a commercial operation would sweep far larger areas for years at a time. The authors of the Nature study note that some aspects of modern collector designs may cause less physical damage than the 1979 machine, so the scale and shape of future impacts are not a simple multiplication of this one.

The study also rests on a single site. Jeffreys says more work is needed to understand how organic matter varies across the wider Clarion-Clipperton Zone, which spans millions of square kilometres with different depths, sediments and food supplies.

The link between food and animals is a correlation. Faunal densities rise and fall with organic matter quality across the three kinds of ground, which fits the idea that a poorer diet keeps populations low, but the study did not test that cause directly. The loss of the nodules themselves, which many animals use as a hard surface to live on, acts on the same ground.

Common questions about deep-sea mining and seafloor recovery

What is the Clarion-Clipperton Zone?

It is a vast abyssal plain in the eastern and central Pacific, between Hawaii and Mexico, mostly four to six kilometres deep. Its floor is covered with polymetallic nodules, potato-sized mineral lumps rich in battery metals, which makes it the main target for proposed deep-sea mining.

How long does the seafloor take to recover from deep-sea mining?

Nobody knows the full answer, but the OMCO test site shows that some damage lasts at least decades. Forty-four years after a single test, the tracks look almost unchanged, fixed animals remain scarce and the sediment still holds less food than the surrounding seabed.

Why does organic matter matter on the deep seafloor?

Almost no light reaches the abyss, so animals there depend on organic particles sinking from the surface. The amount and quality of that material, including nutrients such as omega-3 fatty acids, shape how many animals the seafloor can support and which species live there.

The rules are still being written

The researchers say their results show that long-term monitoring is needed and that changes to the amount and quality of organic matter should be part of how deep-sea mining is assessed and regulated in the Clarion-Clipperton Zone. Because the zone lies in international waters, any commercial mining there falls to the International Seabed Authority, which is still developing the legal, financial and environmental framework that full-scale extraction would require.

“These findings are particularly important as proposals to advance commercial deep-sea mining continue to gather momentum,” Jeffreys said. The paper appeared in Elementa on 2 November 2026, based on samples collected in early 2023, and the SMARTEX project is continuing its work on how abyssal ecosystems respond to and recover from mining disturbance.

Reference: Jeffreys et al., “Long-term impacts of deep-sea mining on sediment organic matter quantity and quality,” Elementa: Science of the Anthropocene, 2026. DOI: 10.1525/elementa.2025.00106

Tags: Clarion-Clipperton Zone, deep-sea mining, Hughes Glomar Explorer, oceans, polymetallic nodules, seafloor