Seven Miles Down | Mariana Trench – The Season Finale

This is as far down as we go.

Nearly seven miles below the surface of the Pacific Ocean, in a place named after the god of the underworld, lies the deepest point on Earth. The pressure at the bottom of Challenger Deep is more than 1,000 times what you feel on a normal day — enough to crush the air from human lungs in a fraction of a second. For most of human history, we assumed nothing could survive here.

We were wrong. And in this season finale, we go all the way to the bottom to find out what's there.

First, we meet the hadal snailfish — a pale, translucent, gelatinous fish that looks like it's falling apart, and is in fact one of the most sophisticated pressure-adapted organisms ever discovered. We break down the molecular secret that keeps its proteins from deforming under crushing weight, why that secret has a hard biological limit at 8,000 meters, and why that limit means even this fish can't reach the very bottom.

Then we look at what feeds a world with no sunlight. The answer is a perpetual slow-motion blizzard — marine snow, falling every second of every day from the world above — funneled by the trench's steep walls into a thick, ancient ooze at the bottom. And we meet the creatures that live in that ooze: including the xenophyophore, the largest single-celled organism on Earth, the size of a mango, absorbing toxic heavy metals from the sediment and hosting an entire microbiome inside and around a single cell.

And then, the hardest part. In 2019, the first person to reach the deepest measured point in Challenger Deep photographed a plastic bag in the sediment. A 2019 study found that 100 percent of amphipods sampled from the Mariana Trench contained plastic fibers in their digestive systems. Industrial chemicals banned in the 1980s are accumulating in trench animals at concentrations comparable to some of the most polluted coastal waters on Earth.

The depth that took machines, engineering, and enormous courage to reach has been reached by a candy wrapper. Effortlessly. Simply by sinking.

This episode ends where the season began: with the question of what life is, and where it ends. The Mariana Trench has an answer. There is no edge. There is no bottom. There is only what we choose to send there.

Secrets of Earth is a nature documentary podcast for all ages, exploring the why and how behind the planet's most extraordinary life.

Transcript

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Welcome to Secrets of Earth. I'm Patrick Versba, your guide into the world's most remote corners. This is our final journey of the season, and it's the deepest one we've taken. Today, we descend into a place that has existed in human imagination for centuries as the absolute limit of the world. We are going to Challenger Deep. If you were to take Mount Everest, the highest point on the surface of the earth, 29,032 feet above sea level, and flip it upside down into the ocean, its peak would be submerged by more than a mile. That is how deep we're going. The Challenger Deep, the lowest known point in the Mariana Trench in the western Pacific, sits approximately 36,000 feet below the surface. Nearly seven miles of water between you and the open sky. At that depth, the water above you exerts a pressure of roughly 15,750 pounds per square inch, more than 1,070 times the standard atmospheric pressure at sea level. This is not a metaphorical weight, it is a physical one. It is eight tons of force pressing on every square inch of your body simultaneously. Any air-filled space, your lungs, your sinuses, any gas bubble would be crushed into nothing in a fraction of a second. For most of human history, we assumed this was the end. It was referred to as the Hadel Zone after Hades, the Greek god of the underworld, because we believed that nothing could survive here. The laws of biology, we thought, simply broke down at this depth. Life required warmth, light, and the absence of crushing weight. We were wrong. As our technology finally allowed us to descend to these depths, first the Bath Escape Trieste in 1960 with Jacques Picard and Don Walsh aboard. Then James Cameron in 2012. Then Victor Vescovo in 2019, who reached the deepest measured point yet. We found something that keeps changing what we thought was possible. The trench is not a graveyard, it is a biome. A strange, pressurized, lightless, cold frontier, but alive. Today, we are going to look at the creatures that have made this home. And we are going to look at what we have done to it in our absence and what that might cost us. To survive in the hadal zone, you have to solve a problem that is, at its core, a problem of chemistry. Imagine everything that makes a living cell work. The enzymes that catalyze reactions, the proteins that carry oxygen in blood, the structures that hold cell membranes together and allow signals to pass between them. All of these are three-dimensional shapes. They fold into specific configurations, grooves and protrusions, locks and keys, and their function depends entirely on maintaining those shapes. This is why temperature matters so much to biology. Heat disrupts the folding of proteins, and once a protein loses its shape, it loses its function. Pressure does the same thing. Under sufficient mechanical force, protein structures deform, enzymes stop working, cell membranes lose their regulated permeability. A heart that cannot maintain the molecular machinery of its cells cannot beat. This is the wall that stands between most life on Earth and the Hadal Zone. It is not just the pressure as a physical force, it is the pressure as a chemical disruptor, a silencer of the molecular machinery of life. The creatures that inhabit the trench have, through millions of years of selection, found ways around this wall. The most iconic deep-sea vertebrate in the Mariana system is the Hadel snailfish, a translucent, gelatinous, pale pink fish that belongs to the family Liparidae. It looks, at first glance, like the most fragile thing imaginable. A limp scrap of wet tissue, almost colorless, with no visible scales, and a body that seems barely held together. It is, in fact, one of the most sophisticated pressure-adapted organisms ever discovered. The snailfish has no swim bladder, the gas-filled sac that most fish use for buoyancy. Gas is incompressible only at low pressures. At hal depths, any gas pocket would be crushed into a tiny, dense residue within seconds. Instead, the snailfish's body is composed almost entirely of watery gelatinous tissue, a structure that is itself essentially incompressible because it's mostly liquid. It doesn't fight the pressure, it becomes the pressure. But the snailfish's deeper secret is molecular. Its tissues are saturated with a compound called trimethylamine N oxide, TMAO. This small molecule works as what biochemists call a piezote, a pressure-counteracting solute that physically stabilizes the three-dimensional shapes of proteins against the deforming force of extreme pressure. Think of it as a molecular scaffold threaded through every cell, holding the machinery of life in place against the crushing weight of seven miles of water. The concentration of TMAO in deep-sea fish scales with depth. The deeper the fish, the more TMAO it produces. This relationship is so consistent that scientists use it as a biochemical depth gauge. But it has a hard limit. Above approximately 8,200 to 8,400 meters, TMAO concentrations reach a saturation point. The cells can no longer produce enough of it to stabilize proteins against the increasing pressure. Beyond this depth, the molecular machinery of fish biology appears to stop working entirely. This is why hadle snail fish are found on the upper slopes and walls of the trench, down to around 8,000 meters, and not at the very bottom of Challenger Deep, nearly 11,000 meters down. They inhabit the Hadal Zone, but even they cannot go all the way. At the deepest depths, the animals are smaller, simpler, built from different molecular strategies. But they are there. In a world with no sunlight, there are no plants, no photosynthesis, no algae covering the rocks, no kelp rising toward the light. The question that biologists confronted when they first confirmed life at Hadal Depths was: What do these creatures eat? The answer arrives from above, in a perpetual slow-motion rain. Every moment of every day, throughout the entire water column of every ocean, a blizzard of organic material drifts downward. It is called marine snow. A mixture of dead plankton, fragments of zooplankton, fish scales, fecal pellets, bacterial aggregates, and microscopic detritus shed by the living world above. In the upper ocean, this material is quickly recycled by organisms that intercept it on the way down. But some fraction always escapes. Its narrow, steep walls funnel organic material down from the surrounding abyssal plain, concentrating it on the trench floor in a thick, fine ooze of sediment. It is, by the standards of the deep ocean, a relatively rich environment. The convergence of thousands of miles of sinking organic material from across the Pacific. The primary scavengers of this ooze are the amphipods, small shrimp-like crustaceans that have evolved to be extraordinarily efficient nutrient processors in a food-scarce environment. They can detect the chemical signature of organic matter, a fallen whale carcass, a large fish that sank from the surface from considerable distances in the dark, and they arrive in enormous numbers when food is available. Life at this depth is governed by a single principle. Patience. With food scarce and unpredictable, the metabolism of hatal organisms slows toward its minimum. Some deep-sea creatures appear to only feed occasionally, cycling through long periods of low-energy waiting. They are not dormant, they are economizing. They have built a biology around the assumption that the next meal is far away in time. And when the snow delivers something large, a whale fall descending through miles of water, the response is immediate and intense. But the Hadal Zone is also home to creatures that go beyond simple scavenging. Organisms that have found a way to eat the sediment itself. Scattered across the gray silt, among the amphipods and sea cucumbers are objects that look like discarded, crumpled pieces of gray tissue. Like someone has dropped dozens of small wet brains on the seafloor. These are xenophiophores. In the world of biology, the xenophiophore is a standing challenge to our assumptions about what a single cell can be. In almost every other context, cells are microscopic, visible only under a lens, measured in microns. Xenophiophores are the largest known single-celled organisms on Earth. A mature individual can grow to four inches across, roughly the size of a mango. It is a single cell, one membrane, one diffuse network of cytoplasm and nuclei spread through a structure large enough to hold in your hand. They survive by absorbing nutrients directly through their membrane, filtering the marine snow and bacterial matter in the sediment without any of the specialized organs that multicellular animals use to feed. They have no nervous system, no heart, no eyes. They perceive nothing. They simply exist, growing slowly through the ooze, processing the chemistry of the deep. But their most remarkable characteristic, and the one that makes them important to the entire Hadel ecosystem, is their relationship with heavy metals. Xenophiophores are extraordinary hyperaccumulators. They absorb and concentrate high levels of lead, uranium, mercury, and other toxic metals from the surrounding sediment, sequestering them within their tissues in concentrations that would be lethal to almost any other organism. What this means for the ecosystem around them is still being worked out. But it appears that by concentrating these metals within their own bodies, xenophiophores act as a kind of geological sink, slowly processing the chemistry of the Earth's crust as it settles into the deepest basins. They are the quiet biogeochemists of the abyss, doing work we are only just beginning to understand, in a place where almost no one has been able to observe them. And beneath them, in the sediment they inhabit, in the structures they create, lives a dense community of microbes, bacteria, and archaea adapted to the specific chemistry that the xenophiophore produces. A single large xenophiophore may host an entire microbiome, a community of organisms living within and around a single cell. A city within a single living thing. There is a powerful and comforting myth that surrounds the deep ocean. It goes something like this. Whatever damage we have done to the surface, the pollution, the plastics, the synthetic chemicals, the deep is safe. It is too remote, too deep, too far removed from the worlds of ships and shorelines and drains. The abyss is a sanctuary of last resort, untouched by human hands. In 2019, Viktor Vescovo became one of only a handful of human beings to descend to the floor of Challenger Deep. He set a new depth record. He collected samples for science. And he photographed in the sediment at the deepest point on earth a plastic bag and a candy wrapper. This was not a surprise to the scientists who had been studying hatal pollution. The trench is a topographic trap, the lowest point in a vast ocean basin that has been receiving the runoff of human civilization for more than a century. Everything that sinks in the Pacific eventually finds its way here. A 2019 paper published in the journal Royal Society Open Science documented something more systematic than a single plastic bag. Researchers collected amphipods from six of the deepest ocean trenches all around the Pacific Rim, including the Mariana. They examined the hindgut contents of each animal. In the Mariana trench samples, every single amphipod, 100% of them, contained plastic fibers or particles in its digestive system. 100%. These are animals living at depths between 7 and 11 kilometers in a place visited by fewer people than have stood on the surface of the moon. They have never been in contact with the surface world. They have never seen a fishing net or a plastic bag. And yet the fibers from human textiles, washed from clothes in washing machines, carried through rivers to the ocean, settling through miles of water, had reached them, had been eaten by them. The PCB contamination is in some ways even more disquieting. Polychlorinated biphanols, industrial chemicals banned in most of the world by the 1980s, were found in deep trench amphipods at concentrations comparable to some of the most heavily industrialized coastal waters in the Northwest Pacific. These chemicals were banned because of their toxicity, their tendency to accumulate in biological tissues, and their persistence. They do not break down. They were introduced to the ocean decades ago, and the deep trenches, with their stillness and their topographic isolation, have been collecting them ever since. The chemistry we introduced to the surface world has settled slowly and permanently into the deepest place on earth. The isolation that we imagined protected the trench from us has instead made it a concentrator of our waste. Life at Challenger Deep has survived the weight of water for millions of years. It is less clear whether it can survive the weight of our chemistry. We have traveled far this season. We have hovered above the seafloor of the Pacific Northwest in a forest of crystal glass. We have drifted through the open blue aside a creature that farms bacteria on its own body, miles below the surface. We have watched an octopus become a sea snake in seconds on a desert of volcanic sand. We have heard the call of the largest animal ever to live on this planet, traveling a thousand miles through the dark. And now we have reached the bottom. The Mariana Trench asks us a question that all of these journeys have been building toward. How do you define the limits of life? For centuries, we drew those limits at the edge of sunlight, at the edge of warmth, at the edge of reasonable pressure. The creatures of the deep have erased every one of those lines. Life is not a thin film on the surface of a hospitable planet. It is a force that has worked its way into the volcanic veins of the ocean floor, into the crushing dark of the hedal zone, into pressures that should silence every biological process. It has found a chemistry where there is no chemistry. It has built a food web from a rain of debris falling through miles of darkness. And now it has found something else down there. The residue of us. Effortlessly, simply by sinking. What we do at the surface matters at the bottom. I'm Patrick Viersba. Thank you for journeying with me through secrets of Earth. The world is full of them if you only know where to listen. I'll see you on the next horizon. Until then, follow our coordinates by subscribing to or following the show. It ensures that you never miss a step into the unknown.