Sampling ferromanganese crusts with the ROV Isis at Tropic Seamount on the JC142 Research Expedition on the RSS James Cook.

Ferromanganese deposits were one target of a recent research expedition to the Tropic Seamount off of Africa's coast.


Mountains hidden in the deep sea are biological hot spots. Will mining ruin them?

In late 2016, scientists aboard the U.K. research ship James Cook arrived 500 kilometers off the coast of northwest Africa, seeking two treasures joined by a curse.

Submerged 1 kilometer beneath the ship, in permanent darkness and crushing pressure, was a dead volcano, the Tropic Seamount. One treasure bloomed on its flanks: an otherworldly, multicolored forest. Curled, pale sponges that looked like potato chips the size of dinner plates grew there, as did spindly white corals festooned with squid egg sacks.

Much of that life clung to rock coated with a black, bumpy skin as bland as asphalt. That crust, about 12 centimeters thick, is the other treasure—rich in the rare minerals that help drive the modern economy. On the Tropic Seamount alone, it might hold enough cobalt to power 277 million electric cars and enough tellurium to build solar panels generating more than half of the United Kingdom's electricity.

The curse: Getting at those minerals might mean destroying the biological riches on top of them. That conundrum motivated the voyage to the seamount. "We're trying to solve the world's demand for critical elements which we need to create a low-carbon, sustainable environmental future," says Bramley Murton, a marine geoscientist who led the expedition for the National Oceanography Centre in Southampton, U.K. "Could we do it in a way that would have a minimal impact?"

As scientists map and explore the estimated 30,000 major peaks beneath the ocean's surface, the tension between conservation and exploitation is sharpening. More than 2 centuries ago, Alexander von Humboldt documented how climate and other physical conditions shape the ecology of terrestrial mountains. Now, marine researchers are exploring how the unique current patterns near seamounts nurture their extraordinarily rich ecology—and coat them with some of the planet's richest mineral deposits.

A charted peak

The Tropic Seamount off of northwest Africa’s coast is one submerged peak getting attention from marine geologists and biologists. Overall, however, researchers have visited relatively few of Earth’s estimated 30,000 seamounts, which come in a vast array of shapes and sizes. Many sit deep in largely unregulated international waters.

23 kilometers Areas with documented ferromanganese crusts 2900 1250 24 450 900 1000 m Distance from surface to peak Height Tropic Bear Cadamosto Atlantic Atlantic Atlantic Conical Pacific Davidson Pacific Bowie Pacific Bounty Pacific Name Ocean 1100 3000 m 2000 1500 600 2280 3000 3950 –968 meters –4323 Bathymetry Atlantic Ocean Sizing up seamounts

The emergence of deep-diving robots, precision sonar, and other technologies that give researchers an unprecedented ability to study seamounts are also enabling moves toward commercial exploitation. In recent decades, for example, fishing boats equipped with deep-diving nets have targeted species that school on submerged ridgetops, sometimes with little regard for the long-term ecological consequences. Now, seamounts have caught the eye of people interested in using sophisticated machines to mine deep-sea habitats, including hydrothermal vents and abyssal plains, that hold precious minerals.

Although seamount mining is mostly just a concept for now, many researchers believe it is only a matter of time before it becomes a reality. The implications are looming large in the minds of many scientists, says Cindy Van Dover, a biologist at the Duke University Marine Lab in Beaufort, North Carolina. "Any paper you read now on deep-sea science," she says, "has something about how this relates to deep-sea mining."

Like many of the thousands of seamounts dotting the oceans, the Tropic Seamount started as a volcano, 120 million years ago. It lies at the southern tail of a chain that includes submerged peaks as well as the Canary Islands off the coast of Western Sahara. The seamount rises 3 kilometers from the ocean floor and is topped by a plateau 50 kilometers wide, 1 kilometer below the sea surface. Above ground, it would rank among the world's 100 tallest mountains.

Viewed through cameras mounted on Isis, a car-size robotic submarine launched from the James Cook, the seamount is a mixture of sand and dark gray rocks. Much of its surface is encrusted with minerals that precipitated out of the seawater over eons, coating the lava at the excruciatingly slow rate of 1 centimeter or less every 1 million years.

That coating has caught the eye of prospectors. Called ferromanganese crust, it can contain high concentrations of cobalt, tellurium, and rare-earth elements used in electronics such as wind turbines, batteries, and solar panels. By one estimate, seamounts in just one chunk of the North Pacific Ocean could hold 50 million tons of cobalt—seven times the worldwide total that's economical to dig up on land. Such estimates arrive at a time when the International Energy Agency in Vienna is warning of a possible cobalt supply crunch by 2030, caused in part by the growing production of battery-powered cars.

Companies hoping to extract those metals from the seabed are focusing first on abyssal plains. Those flat expanses of the deep ocean floor can be littered with potatolike nodules rich in nickel, copper, and cobalt. They are also looking at hydrothermal vents that spew mineral-laden water, creating thick crusts and fantastical rock chimneys. Seventeen companies have permits to explore for minerals in one abyssal region, the Clarion-Clipperton Zone in the Pacific Ocean between Hawaii and Mexico. And in 2017, Japan became the first nation to conduct large-scale experimental mining of a dead hydrothermal vent off the coast of Okinawa, inside Japan's national waters. But the crusts on seamounts have particularly high concentrations of sought-after metals, making them a tempting target.

The Tropic Seamount was one of two underwater peaks that the 2016 expedition visited. The $5 million campaign, known as MarineE-tech, included scientists from the United Kingdom, Brazil, and Spain. The chief goal was to better understand how and where crusts form on seamounts—information that could help prospectors determine which ones are likely to hold minerals.

The Isis, a deep diving robotic submarine, is used to explore and take samples from seamounts.


Scientists spent 6 weeks bobbing above the seamount. They dispatched a torpedo-shaped underwater drone equipped with special sonar to map it. They used Isis to collect chunks of the crust, drill 100 rock cores, and take photos and videos of sea life. They measured water chemistry and currents with equipment tethered to the bottom or lowered from the ship.

The researchers ended up with a fine-grained portrait of the mountain, detailed enough to show every square meter. The data on currents and chemistry showed how water swirls around the seamount. The core samples revealed distinct, fine layers of crust that chronicled how ocean conditions differed in the past, just as tree rings reveal past climate patterns.

The results yielded new insights into how the mineral crusts form. The seamount is buffeted by the Canary Current sweeping southwest along the African coast and by the tide roiling back and forth each day. The currents speed up as they detour around the mountain, sweeping the rock clean of sediment and producing a surface on which minerals can build up. Near the summit's center, the current swirls sand and grit over the surface, wearing down the crust and keeping it thin. But thicker crust forms at the summit edges and on the steep flanks, where the currents moderate.

Ultimately, Murton says, researchers hope the data the team gathered on currents and geology at the Tropic and another seamount off the Brazilian coast, the Rio Grande Rise, will help them create computer models that can predict the locations of seamount crusts. Such modeling could reduce the need for costly and time-consuming drilling, says James Hein, a marine geologist and geochemist at the U.S. Geological Survey in Santa Cruz, California, who consulted for the Tropic expedition. "Any way that one can look at the current patterns and suggest about how crusts might be distributed is very important."

The problem, Murton notes, is that what his team learned on the Tropic Seamount puts mining and conservation on a collision course. "The conditions that seem to favor the growth of the crusts," he says, "also seem to favor the colonization by a lot of corals and sponges."

The Tropic Seamount's thin crust (black) could hold key minerals.


Seamounts cover roughly the same area as Russia and Europe combined, by one estimate, making them one of the planet's largest habitats. The peaks have long been known as oases for sea life. The swirl of ocean currents can concentrate nutrients and free-floating larvae. The craggy protrusions above the thick muck that covers the ocean floor serve as resting and hiding spots for some animals but disrupt the movement of others, making them easier prey. Schools of fish—brick-red orange roughy, silvery pelagic armorheads, and goggle-eyed black oreos—often congregate at seamounts, as do sharks and tuna. Some migratory humpback whales appear to use them as navigational markers, spawning grounds, and resting spots. Seabirds gather above them, and myriad corals and sponges cling to their rocky surfaces, creating ample cover for other creatures.

Yet those deep oases remain poorly understood. In contrast with ecosystems on land, scientists get only glimpses of life on seamounts—the contents of nets hauled up from the deep and videos of whatever appears in the circle of light cast by a submarine. But thanks partly to growing interest in mining, researchers are assembling a more detailed portrait of seamount biology. Nations and companies are steering money toward more biological research as they discuss how to regulate deep-sea development. The mining push has also attracted academic researchers. "Everybody I can think of has somehow jumped onto doing something" regarding mining, Van Dover says.

Interest in seamounts is particularly high in countries that either host companies interested in deep-sea mining or are considering allowing mining in their national waters. In 2018, the Chinese research ship Kexue (meaning "science") spent about 1 month surveying the Magellan Seamounts near the Mariana Trench, which several nations see as a potential source of industrial minerals. Brazilian researchers teamed up with Murton's MarineE-tech project to examine an area in international waters where the country has a preliminary mining claim. Japanese scientists sent robots to survey seamounts that might be ripe for mining. In late July, the International Seabed Authority (ISA) in Kingston, a part of the United Nations that governs deep-sea mining in international waters, released 18 years of environmental data gathered by companies pursuing mining claims, including on seamounts.

Some scientists have a different focus: building a list of seamounts where mining should be restricted or banned. Lea-Anne Henry, a marine ecologist at the University of Edinburgh, scanned 19,000 images from the Tropic Seamount, looking for species that could make it a candidate for U.N. designation as an ecologically important ocean habitat. She cataloged an exotic menagerie: fields of glass sponges—animals that use silica to build intricate, glasslike structures; soft-bodied octocorals, including pink umbrellas on slender stalks (Metallogorgia melanotrichos) and multihued fans (Corallium tricolor); and the intricately sculpted homes of xenophyophores, single-celled organisms that build "shells" from sand. The finding was enough to convince Henry that much of the Tropic Seamount warranted protection. "My motivation is just to identify these areas before it's too late," she says.

A newly hatched small-spotted catshark clings to a sea fan on the Ullastres Seamount off the coast of Spain.


Biological oceanographer Les Watling and colleagues at the University of Hawaii in Honolulu are doing much the same for seamounts on the other side of the planet, in the central Pacific. They documented 91,000 specimens in videos taken during 170 deep-sea expeditions between 2015 and 2017 by the Okeanos Explorer, a research ship belonging to the U.S. National Oceanic and Atmospheric Administration. Watling wants to create maps of species that are indicators of vulnerable ecosystems—such as coral and sponge species that can easily be damaged and grow slowly. He hopes doing so will compel governments and mining companies to take those species into account before allowing any seamount mining to begin. "So far, indicator species are pretty well everywhere, and they occur at all depths," he says.

Still, most of what's known about life on seamounts is a catalog of species, not the detailed interactions that underpin an ecosystem. As a result, it's hard to predict what will happen if part of a seamount gets mined, says Malcolm Clark, a marine biologist and seamount expert at New Zealand's National Institute of Water and Atmospheric Research in Wellington. "We can describe what's there," he says. "What we can't really understand very well is what's going to affect the broader sustainability of the system when we take a bit of it away."

The design of seamount mining equipment is closely guarded by competing countries and companies. But it could work much like equipment being tested for hydrothermal vents: enormous, remote-controlled machines that resemble bulldozers, equipped with toothed wheels designed to grind the crust into bits that can be carried to the ocean surface for processing.

Although no seamount has been mined yet, scientists point to the damage from deep-sea fishing to underscore why they worry this heavy machinery would do irreparable damage. In the late 1990s, Australian scientists documented devastation from nets dragged across seamounts near Tasmania to catch orange roughy. Hard corals had been wiped out, and the sheer mass of life on the mountains was half that on nearby ones too deep to be fished. Fifteen years after trawling was halted on some New Zealand seamounts, Clark and other researchers found little evidence of recovery.

A more recent study, of seamounts near the Northwestern Hawaiian Ridge that were once heavily trawled but have been protected for more than 30 years, had a more encouraging conclusion. Some "showed multiple signs of recovery, including corals regrowing from fragments," biologist Amy Baco-Taylor of Florida State University in Tallahassee and colleagues reported last month in Science Advances. However, she and others note these seamounts rise closer to the ocean surface—where organisms might grow more quickly—than those studied near Australia and New Zealand.

Unlike fishing, mining is also expected to produce copious amounts of sediment, either stirred up by digging or created when muddy waste is dumped back into the sea after ore is lifted to the surface and processed. Little is known about how big such plumes might be, but computer models suggest silt released in some parts of the ocean could drift 10 kilometers or more before settling back onto the sea floor, potentially smothering creatures living there.

Experiments by Murton's team hint that the sediment damage on seamounts might not be as severe as some fear. In one test at the Tropic Seamount, a submarine sucked sediment from the top of the seamount and spewed it a few meters up into the water column. Nearby instruments measured how cloudy the water became near the mountain's surface while an aquatic drone did the same 1 kilometer away. The results showed most of the sand and silt quickly settled out, says Jez Spearman, the experiment's leader and a marine sediment transport expert with HR Wallingford, a private research firm in the United Kingdom. "Even when we've thought about real mining, the plume's not going very far," he says. "It's a few kilometers."

Where sediment does fall, it may do less damage than feared. Mikhail Zubkov, a microbial biologist at the Scottish Marine Institute in Dunbeg, took ground-up bits of the mineral crust from the seamount and sprinkled it on phytoplankton. He expected much of the algae would die when exposed to the toxic metals in the crust, such as copper. To his surprise, however, "We didn't kill them," he says. That could be because the crystal structure of the minerals prevents the toxic elements from escaping, Zubkov says.

Those findings, which haven't been published, aren't the final word. Clark and his New Zealand colleagues in June mounted a bigger test, kicking up a sediment plume on the Chatham Rise, an undersea plateau east of New Zealand. Next year, the scientists will return to see how nearby organisms fared.

Such findings could help nations decide how to regulate seamount mining. The rugged terrain on seamounts and the challenge of peeling off a thin layer of crust mean they won't be exploited anytime soon. But Russia, Japan, South Korea, and China have all gotten mining permits to explore seamounts in the northwestern Pacific. A Brazilian company is looking in the Atlantic Ocean off the coast of South America. Hein says the concentrations of valuable metals on seamounts will prove irresistible and expects mining to begin by the 2030s. "Eventually," he says, "it's going to happen."

If seamount mining does happen, many of the digs are expected to be located in the high seas—a kind of legal no man's land beyond the reach of national laws, covering nearly 45% of Earth's surface. ISA, which includes representatives from more than 160 countries, is hammering out mining regulations for the high seas.

Environmentalists and some scientists worry the agency is rushing. In July, a group of 29 deep-ocean researchers from Europe, the United States, and China warned that permitting mining before the deep sea is better understood would put "the overall health of ocean ecosystems under threat." The agency has defended its approach. In July, ISA Secretary-General Michael Lodge issued a statement saying that the agency "places protection of the environment and benefit to humanity at the front and centre of its mandate."

Clark serves on a technical committee advising ISA on mining. He agrees that too little is known about deep-sea ecosystems today to forecast the environmental impact of seamount mining. But instead of blocking all mining, Clark says the best approach might be a gradual one, allowing mining on a small scale and watching to see what happens to the surrounding marine life. "We probably won't learn enough about the impacts of how the system might respond until we do a bit of mining."

No mining company is yet targeting the Tropic Seamount. The sea creatures illuminated by Isis's spotlight have returned to darkness, grasping onto the precious minerals as they have for centuries, undisturbed for the moment.