On 8 March 2018, at 1.20am, Malaysian Airlines flight 370 veered off its scheduled route from Kuala Lumpur to Beijing. An hour later, military radar spotted the plane heading west over the Andaman Sea. Six or seven hours later, it is presumed to have crashed somewhere over the southern Indian Ocean, one of the least studied bodies of water in the world.
Just how little we knew about this part of the ocean became clear during the subsequent search for the missing aircraft. Before a proper underwater search could even begin, a vast stretch of seafloor had to be mapped. Over the next three years, a team of ships from Australia, China and Malaysia scanned the bottom with a combination of submersible robots and ship-borne sonar. Together, they charted a swath of ocean roughly 1,500 miles long and 150 miles wide, encompassing an area the size of France. The maps produced from these scans revealed a lost world, full of undersea canyons, crevasses, volcanic plateaux and a single, enormous cliff taller than the Swiss Alps. Even the abyssal plains, thought to be some of the flattest areas on the planet, were home to previously uncharted hills.
If you want to follow in the footsteps of the great explorers, forget the moon and Mars: the ocean floor is where the real action is. The deep ocean, the part that’s deeper than 200 metres, covers about 66% of the Earth’s surface. Most of it has never been surveyed in detail. Even less has been seen up close. If the current rate of observation continues, a complete visual survey of the ocean floor will take about 5m years.
While the deep ocean remains out of sight and out of mind, its effects are omnipresent. The deep ocean, with its immense ability to store heat, functions as the planet’s thermostat. The oceans also absorb roughly 30% of the carbon dioxide we pump out into the atmosphere, and generate 80% of our oxygen (though about half of this stays in the ocean itself). As physicist Helen Czerski brilliantly demonstrates in her 2023 book The Blue Machine: How the Ocean Works, the oceans are the motor behind that huge engine of circulating heat and water vapour we call the weather.
The oceans regulate the climate, make our air breathable, process our emissions and recharge our rivers. But the ways in which they do all of this are largely hidden from view, and so we hardly give them a thought. This ignorance extends to the ocean’s creatures, especially those that dwell in the true deep.
The deep ocean is the largest ecosystem on Earth. It is also in many ways the most extreme, home to crushing pressures, extremes of heat and cold, and a near total absence of sunlight. Animals inhabiting this midnight world tend to be equally extreme. It is a menagerie that abounds in superlatives: the largest, the oldest, the blackest, the most luminous. But those are only the ones we know about. Most of the animals dwelling in the benthos, the true deep, remain unknown to science. Virtually every scientific expedition to reach this zone of darkness returns with new species in tow. In the past year, scientists have discovered more than 1,100 new marine species. Among them are a ghost shark (not really a shark), a ping-pong ball sponge (which does look like a cluster of ping-pong balls), a number of luridly coloured worms and a floating something that resembles a tiny jet plane made out of pale pink jelly, and which scientists have not yet been able fit into any of the primary categories of animal life.
Certain regions of the ocean floor are home to even more hostile conditions. Vents of superheated water, packed with toxic chemicals, would spell death in our world. In the deep, however, they are home to some of our planet’s most vibrant, and alien, animal communities. More than any hypothetical discovery on Europa or Mars, they may offer our best glimpse into the origins of life on Earth. But there is a growing possibility that these rare oceanic oases may be destroyed before anyone gets a chance to study them.
For centuries, the oceans have been seen as a pool of virtually infinite resources. In the 19th century, whales were hunted nearly to extinction for their oil and baleen. Today, this same extractive attitude towards the sea persists. Every year, fishing fleets pull an estimated 80 million tonnes of seafood out of the oceans. This industrial approach to fishing is now being expanded into ever deeper waters. For the moment, however, the true abyss remains mostly untouched, at least as a commercial resource. That may be about to change.
For over 50 years, would-be industrialists and entrepreneurs have floated the idea of mining the ocean floor, but without much happening in practice. But in our search for new sources of metals needed for batteries and microchips, we may now be on the cusp of destroying the world’s largest – and strangest – ecosystem before we get a chance to understand it.
For centuries, it was thought that nothing lived in the deep. For much of the 19th century, naturalists were convinced that life was incapable of surviving below 1,800 feet (550 metres). This “azoic” theory of the deep persisted even after soundings turned up sea lilies – delicate, flower-shaped organisms related to starfish – living on the bottom of Norwegian fjords. It took the Challenger expedition, a British Navy-sponsored research project which circled the world between 1872 and 1876, to finally dispel the myth of the ocean floor as a lifeless waste.
The Challenger expedition also resolved one of the great, lasting mysteries of early oceanography: the true depth of the seas. Dropping a plumb line into the ocean 300 miles east of Japan, the researchers sounded a depth of 27,450 feet (8,370 metres), the deepest point ever recorded at that time. Later voyages would show that this hollow was part of the Mariana Trench, one of many deep gashes creasing the ocean floor along the junctions between tectonic plates.
In addition to finding one of the ocean’s deepest points, the Challenger team compiled the first map of ocean depths. Their motivations were practical as well as scientific. Their voyage took place during the first great information revolution, powered by the telegraph. Detailed knowledge of the ocean floor was needed in order to lay telegraph cables between continents. But while the expedition discovered the true immensity of the oceans, it took another half-century before anyone tried to figure out what actually lived in the several vertical miles of water they had uncovered.
Organisms that dwell in the deep tend to be either delicate, skittish, or both. Adapted to life under extreme pressures, they often disintegrate and dissolve on the way to the surface. To really understand what is going on in the middle of the ocean column, one has to go there in person. The first person to try this in earnest was the explorer and ornithologist William Beebe. In 1930, Beebe had himself lowered 1,400 feet (430 metres) into the seas off the Bahamas in a “bathysphere”.
Invented by Otis Barton, who accompanied Beebe on his descent, the bathysphere was one of the ungainliest vessels in the history of exploration. A two-tonne sphere made out of one-inch-thick steel, it had to be lowered into the ocean on a winch. A single three-inch window made out of fused quartz allowed Beebe and Barton to look out into the twilight world of the benthos. This tiny porthole was enough for the two to see a multitude of bioluminescent creatures flitting through the dark. Through the crystal window, Beebe and Barton watched glowing jellyfish, silver, pincer-lipped eels, tremulous pelagic snails and palm-sized lantern fish dangling green lures from their mouths.
Larger creatures lurked in the distance. Some shone with their own yellow and purple light, others were as large as six metres long, and belonged to no recognisable group. Beebe gave these cryptic beasts names regardless, many of which could have some straight out of a Wes Anderson film. There was the abyssal rainbow gar, the pallid sailfin, the five-lined constellation fish and, finally, the untouchable bathysphere fish, which no one has caught or seen since, and which now gets dismissed by scientists – perhaps unfairly – as a hallucination.
Over the decades that followed Beebe’s reports, it gradually became clear that the twilight zone – the band between about 200 and 1,000 metres deep, where the sun’s light is too feeble to support photosynthesis – is home to an enormous diversity of life. Many of the creatures inhabiting these middle waters are made of jelly. Some, like the siphonophores, live in gigantic, ribbon-like assemblages whose cooperative behaviour blurs the line between individual and colony. Others, like the gossamer worms and giant jellyfish, are lone predators. Without sunlight, these odd, diaphanous creatures rely on their bioluminescence to communicate, hunt and mate. Some go the other way, becoming as dark as can be to avoid getting caught in another’s jaws.
Once thought to be a mostly lifeless desert, the twilight zone, it now appears, hosts numbers of fish in the quadrillions, or millions of billions. Just one denizen of this zone, the bristlemouth, a crayon-size fish with a miniature monster’s maw for a face, is the single most abundant vertebrate on Earth.
Tiny, translucent and beyond counting, these creatures of the mid-water would seem to be one of nature’s footnotes. In fact, they are another crucial gear in the hidden motors keeping our planet in balance. Each day, the animals of the twilight zone take part in what may be the planet’s largest migration – one that is vertical, rather than horizontal. Every evening, they rise from the depths, where the darkness protects them from predators, to the top layer of the ocean, where they can feast on photosynthesising plankton and other marine organisms that derive their energy from the sun. When morning approaches, they sink again. In the process, writes the journalist Susan Casey in her engrossing 2023 book The Underworld, they pull an immense amount of carbon from the top of the ocean into the depths, “an estimated 4.4bn tonnes each year, the equivalent of America’s total annual emissions”.
The world of the mid-water was a virtual blank for marine biology until the advent of scuba diving in the 1940s and 50s. In the 1970s, a group of marine biologists working out of the University of California pioneered blue-water diving, a kind of underwater “spacewalk” in which the bottom of the ocean is out of sight. Although disorienting and dangerous, this allowed scientists to penetrate further than ever before into the twilight zone of the ocean. But to truly understand how life operated in the vast part of the ocean that lives in total darkness, they had to go deeper still. For this, they needed special craft.
Beginning in the 1960s, specialised submersibles unlocked the world of the deep ocean for science. In 1977, the Alvin, a staffed craft introduced by the US navy, made one of the great discoveries in the history of marine biology. While diving in the Pacific Ocean 250 miles north-east of the Galápagos Islands, scientists spotted an unexpected cluster of animals grouped together on the seafloor. These included dense groves of tall, vertically growing worms with scarlet plumes and gigantic, pale clams huddled together in total darkness. Nearby, shimmering streams of warm water, loaded with foul-smelling sulphur, issued out of cracks in the frozen lava of the ocean bottom.
A subsequent dive in 1979 off the southern end of the Baja peninsula uncovered an even stranger type of underwater eruption. There, at the bottom of the Gulf of California, Alvin spotted a number of black chimney-like structures spewing streams of superheated water out of their mouths, so loaded with dissolved minerals that they looked like smoke. At 350C, the fluid coming out of these “black smokers”, as they were nicknamed, would be hot enough to melt lead on land. And yet, as in the Galápagos, they were host to a similarly thriving ecosystem.
The community of animals that gathered around these vents seemed to violate all the known rules of ecology. Until then, it was thought that all living beings ultimately received their energy from the sun. For those few animals that made their home in the bottommost parts of the ocean, that energy arrived in the form of “marine snow”. This is a polite term for what is in fact a steady rain of mucus, droppings and tiny plankton corpses, which is the primary way organic matter from the top of the water column makes it into the deep.
Marine snow is indeed the main food supply for the midnight, abyssal and hadal (or deep and super deep) zone of the ocean, but not the dense animal communities spotted by the Alvin in the Galápagos Rift and East Pacific Rise. So where did these worm-and-clam super cities get their food? From the “black smoke” of the superheated vents themselves. Undersea vents can be hot enough to melt metal, or corrosive enough to dissolve skin. Yet they also abound in chemical energy. Bacteria have evolved to tap into this chemical energy, powering their growth with exotic foods such as metal ions, hydrogen and hydrogen sulphide.
Where bacteria grow, others follow. Space is at a premium around the black smokers. The chemicals in these vents are what sunlight is to a rainforest canopy; everyone crowds in for a taste. Although worms predominate, crustaceans and molluscs also find their place in this exuberant, underwater Shanghai: there are giant, sulphide-slurping clams, blind, heat-loving shrimp, yeti crabs, so named because of their wildly hairy appendages, each hair of which is home to millions of partner bacteria.
Although scientists have known about sulphur-and-metal rich black smoker vents since 1977, in 2000 undersea explorers identified an entirely new form of hydrothermal vent community, powered by totally different energies. Located south of the Azores, smack in the middle of the Atlantic, it was nicknamed the Lost City by its discoverers, for its elegant spires of white and grey calcium carbonate, rising out of the seafloor near the Mid-Atlantic ridge.
The animals living around the Lost City thermal vent are quite different from those around the other black smokers: much smaller, and usually transparent. The food source of this “subtle fauna”, in Casey’s felicitous phrase, is bacterial, but the chemistry is different. The Lost City vents form through a chemical reaction between sea water and the oceanic crust. Compared with black smokers, they are relatively cool, and alkaline rather than acidic, and their waters are full of dissolved calcium, hydrogen and methane. At the vents, these compounds combine to generate some of the basic building blocks of life, leading some scientists to speculate that life itself may have arisen in such a place.
Proving that life started in a deepwater alkaline vent will be difficult, however, since we have nothing to compare it with. So far, the Lost City is the only known vent system of its kind. There may be limited time to find more, as the ocean bottom is threatened by the prospect of undersea mining.
In 2017, the International Seabed Authority (ISA), the intergovernmental body that regulates the exploitation of the seabed in international waters, granted Poland, according to Susan Casey, an “exploration contract for hydrothermal vents along the Mid-Atlantic ridge” –a stake that includes the Lost City itself. The Poles have yet to realise their claim on the seabed, but many other countries and companies have since jumped into the ocean-mining fray. Much of their attention has focused on a remote patch of the north Pacific in between Mexico and Hawaii, known as the Clarion-Clipperton fracture zone. This area is particularly rich in manganese nodules – metal-rich rocks that dot the sea floor like sprinkles on a cake.
Manganese nodules have long been known as one of the deep ocean’s oddities. They were first observed by the Challenger expedition, whose dredges also brought back a number of“very peculiar black oval bodies”, according to expedition scientist Charles Wyville Thomson’s description. Chemical tests revealed that the stones, which ranged in size from gumball to grapefruit, were packed with metal, notably manganese, iron, copper, cobalt and nickel.
The idea of exploiting these metallic potatoes has been around for several decades. Today, as marine microbiologist Jeffrey Marlow writes in his richly informative new book The Dark Frontier: Unlocking the Secrets of the Deep Sea, the metals that these manganese nodules contain “are critical for the batteries and computer chips whose enhanced use would help wean us off fossil fuels”.Proponents of undersea mining claim that this makes them vital for continuing the global transition to green energy.This has made the nodules the target of an incipient gold rush.
Found almost entirely in international waters, manganese nodules seem ripe for the picking. Only the ISA stands in the way of potential exploitation, and it has proven itself to be fairly approachable. According to Susan Casey, in recent years, the agency “has granted 31 mining exploration contracts”, which cumulatively cover an area the size of Alaska. Nineteen of these contracts are specifically for nodule mining, while the remaining 12 would, in Casey’s words, “allow miners to investigate scalping the tops off seamounts and grinding up hydrothermal vents”.
The Clarion-Clipperton zone, one of the world’s great wildernesses, is now threatened with a motley of international mining concerns, though until now, none of these entities has progressed past the exploration stage. In their books, Casey and Marlow both take on the question of nodule mining as emblematic of the threats facing the deep ocean. As a member of an expedition tasked with studying the region’s biology, Marlow ventures into the zone himself, returning with a picture of an area harbouring extreme species diversity, with 90% of recovered species new to science. The zone is no Great Barrier Reef, though: its creatures are mostly tiny filter feeders, such as sponges and corals, who make their homes on the surfaces of the nodules themselves. The surrounding sediment, meanwhile, teems with worms.
If the manganese nodules are stripped away by miners, all these creatures will perish. As marine biologist and science writer Helen Scales points out in her beautifully written 2024 book What the Wild Sea Can Be: The Future of the World’s Ocean, these slow-growing lumps – it can take them as long as 3m years to form – “won’t return for millennia”, and the “unique, biodiverse ecosystems they support will be lost – the ghostly-white octopuses and tiny ‘water bear’ tardigrades, the lemon-yellow sea cucumbers, the delicate corals and sponges, plus thousands of other, as yet undiscovered life forms that inhabit the nodule fields”.
There is a commercial case to be made for preservation. The deep ocean is a reservoir of biological diversity. This means that it is a great library of unknown and untested chemical compounds. The creatures of the deep ocean have some of the most extreme adaptations in the animal kingdom, and some of the enzymes and proteins they use to thrive in the deep have already been harnessed by big business. Among a number of products derived from undersea sources, Marlow lists “Fuelzyme, a patented enzyme that helps turn agricultural waste into biodiesel, collected from parts unspecified; and Yondelis, an anticancer drug made using the biomolecules of a type of Caribbean sea squirt”.
Whatever patents remain to be found in the deep ocean, it is the deep’s sheer remoteness that makes the best case for conservation. On a planet that has lost most of its great biomes, from the old-growth forests of Europe to the prairies of the Americas, the deep ocean stands out as one of the few environments that has not yet been ransacked by humankind. The zone, and other abyssal plains like it, are places of profound stillness and slowness, whose heartbeat is measured in the millions of years. The mind recoils at them being despoiled.
Of course, the deep ocean has never been truly cut off from the surface world. Entire communities of abyssal and hadal organisms depend on food from higher up in the water column. This usually arrives in the form of dead bodies. An entire coterie of scavengers has evolved to feast on the scattered corpses of fish and whales.
These animals are joined by a host of even more specialised nutrient recyclers such as Osedax, the bone-eating snot flower worm, which gains its sustenance by drilling into whale bones to extract the bits of grease and marrow within. Recent genetic work suggests that these bone-worms may be even older than whales, and may have been drilling into bones when giant predatory lizards, the plesiosaurs and mosasaurs, ruled the seas. This means that these humble undersea worms evolved in a world of reptiles, outlasted their extinction, and made it through the entire age of mammals. Some day, a new community of animals may evolve that is able to process our refuse; maybe they too will outlive the age of humanity.
Each of the five major mass extinctions in Earth’s history were accompanied by a collapse of marine ecosystems, and each time they bounced back. They have been through asteroid impacts, showers of acid rain and periods of cold so profound the seas froze all the way to the equator. To the ocean, humans aren’t the end of history. We’re just another, potentially brief, period of strife.
In her moving memoir of grief and parenting after the sudden death of her husband, What Did the Deep Sea Say?, Marion Coutts remarks that “the sea operates in cosmic-historic time and those who live against its borders feel it first, but first or last, we all will feel it”. The oldest whales live to be over 200 years old. Greenland sharks, slow-moving, ghostlike predators of the deep ocean, live past 400. Off the coast of Hawaii, there are branches of cold-water corals that are more than 4,000 years old. In the middle of the South Pacific Gyre, under the seafloor, Japanese scientists found microbes that had been sleeping for the past 100m years.
As to the age of the strange ecosystems populating the hydrothermal vents in the true dark, it’s anyone’s guess. Even if all other life on the planet is extinguished, they will probably go on, fed by the volcanic heat of the Earth itself. Seldom visited, barely understood, they are our origin, and if life on Earth and in all the seas were to truly die out, they would be the place where it would start again.
