When the bathyscaphe Trieste reached the deepest point of the Mariana Trench in 1960—the first crewed vessel to see the lowest known point of the ocean—the two humans aboard reported that there was nothing but a flatfish. They were wrong.

To a eukaryotic imagination, the bottom of the ocean is a hadal zone, something hellish, abyss-like, barren. A lowly fish notwithstanding, the deeper one goes, the less there seems to be. Vessels that have gone to Challenger Deep—the bottom of the Trench and the deepest place in all of Earth’s seas, a site about 35,000 feet below the surface of the Pacific—have seen little in the way of complex life. That makes sense—most life is dependent on energy and carbon from photosynthesis, and light doesn’t penetrate well enough to support it below about 600 feet. The remaining 34,400 feet lie in relative or complete darkness.

Those divers can be admonished if not forgiven for focusing on things like “complexity” or “cell membranes” or “spines.” No matter how deep the submersible goes, no matter how empty the waters seem, there will always be something there. As far as the ocean is concerned, the deepest depths can contain more varied, diverse bacterial life than even the more pleasant, habitable areas of waters near the surface.

In the ocean, bacteria account for almost as much physical mass as animals do—and the vast majority of Earth’s total animal biomass is found there, rather than on land. It’s not too much to say that, depending on the metric, bacteria are the most successful and diverse organisms in Earth’s history, occupying essentially any niche imaginable. Despite their invisibility and their single-celled nature, bacteria account for as much as 15 percent of the biomass of the entire world, second only to plants, the planet’s true conquerors, who make up 81 percent.

Bacterial life is unbound, far more free than any plant or animal. They are found on the tops of mountains, in the thinnest parts of the atmosphere, and at the most crushing depths of the ocean. For bacteria, the biotic difference between the mountain’s peak and the deep ocean shrinks considerably. “When [Tenzing Norgay and Edmund Hillary, the first climbers confirmed to have reached Everest’s summit in 1953] planted their flags on the highest mountain, they set them in snow over the skeletons of creatures that had lived in the warm clear ocean that India, moving north, blanked out,” John McPhee wrote in 1981. “Possibly as much as twenty thousand feet below the seafloor, the skeletal remains had formed into rock. This one fact is a treatise in itself on the movements of the surface of the earth. If by some fiat I had to restrict all this writing to one sentence, this is the one I would choose: The summit of Mt. Everest is marine limestone.”

Atop that limestone, microbes persist too. The bacteria that grow on top of the mountain still share some commonalities with those that grow on the bottom of the ocean and would likely have something to talk about if they ever met. Among the major differences between the two groups, at least metabolically, is that on top of Mount Everest bacterial cells are evolved to live in direct sunlight, while bacteria on the bottom of the ocean are not.

*

The two men aboard the Trieste swore it was some kind of fish. “Even as I saw him, his two round eyes on top of his head spied [us, and] . . . extremely slowly, this flatfish swam away,” reported Jacques Piccard. “Moving along the bottom, partly in the ooze and partly in the water, he disappeared into his night.”

Maybe it was a sole, or a flounder, or something. They were never quite clear, and admitted they were not biologists so really it was anyone’s guess. Decades of arguments ensued about whether it was possible for an actual, living vertebrate to persist at a pressure of 1,100 times that of the surface, or the rough equivalent of “one hundred elephants standing on your head,” as one research institute put it. Consensus in recent years has circled around two key facts that disallow at that depth any kind of life with a spine, the first being those hundred elephants. The temperature at the bottom of the ocean, being about 1℃, can be considered a zwischenzug.

The second is that bony marine fish are osmoregulators, meaning they maintain a precise salt concentration in their bodily fluids, just like warm-blooded mammals keeping their bodies heated to specific temperatures no matter how hot or cold it is outside. That maintenance requires specific osmoregulating molecules, which not only keep salt concentrations on target but also stabilize proteins and other tissues.

But the deeper the fish go, the more of these stabilizing molecules they need. At around 27,000 feet, about the depth of the deepest detected living fish, the amount of osmoregulating molecules needed to keep tissues stable would tip overboard and make a fish’s bodily fluids hyperosmotic. A fish past that depth would become untenably saltier than its environment, and, as water forced its way into its body, would finally explode.

So there are likely no real fish below 27,000 feet. The Trieste could have seen something, but it wasn’t a fish. It was probably something more prosaic, a sea cucumber perhaps, which are common on ocean floors worldwide and so slightly less noteworthy even at Challenger Deep.

In the decades that ichthyologists were indulging in humanity’s infinite capacity for niche games of this-or-that, other scientists were making discoveries. They found that rather than being barren wastelands where the presence of a lonely flatfish would be earthshaking news, all 35,000 feet of the deepest hadal trenches had healthy and varied bacterial populations. Because of recurring deposits of organic material that bury and rebury communities in the sediment at the bottom of the ocean, those buried communities can even be more diverse in bacterial populations than the comparatively more stable upper regions of water above them.

Sampling the vertical depth of the ocean creates the view of vast communal apartment complexes, smashed together in surprising ways. Bacterial species that prefer air likeActinomarinales dominate near the top levels but live throughout the water column, even at the very bottom, suggesting that cells that prefer air can oftentimes just go without. Unsurprisingly, that same sampling found the air-skeptic Aerophobota—it’s right there in the name—dominating the deepest depths of Challenger Deep, cohabiting with Burkholderia, a generally air-preferring genus of bacteria.

One must admire their hardiness and their openness to change. Bacteria do not have to worry about salt, gravity, or pressure. They have no tissues, bones, or spines. Well, whether they worry or not is beyond the scope of this article. Bacteria can adapt quite well to the saltiness and the hydrostatic pressure of hadal trenches, given their nearly universal ability to grow in any ecosystem and adapt to any environmental pressure. Those that live in the highest pressure zones gain the charming moniker “piezophiles” for their troubles. The microbiome of the human gut is pastoral, scenic even, compared to the hellish environments bacteria inhabit all over the planet. The screaming hot geysers of Yellowstone, hydrothermal vents where the water is literally boiling; the clicking ruins of Chernobyl—there is no niche bacteria cannot occupy.

In my time in the sciences, there has been a recent onrush of interest in smaller, less charismatic forms of life. Maybe in part this ameliorates decades of inattention to the bacterial world, creating something like an inferiority complex. Whole popular-science books have been written in the last few years to announce that, yes, bacteria do matter. I once heard a microbiologist, who was talking about the then-new cadre of scientists using CRISPR gene editing to modify human cells, pithily refer to this class of researchers as “eukaryotic gene jockeys.” It’s cool to edit higher-order genes—creating whole new types of medicines, avenues for research, and freaky petunias that glow in the dark—but microbiologists had been splicing bacterial genomes for decades, using far more laborious processes, and no one seemed to give a damn.

“Well, why should they,” one might have said 10 years ago. “What’s in a bacteria? Especially one at the bottom of the ocean?” Everything, about as much as is in any cell on Earth. Every cell solves a problem—how to pass a message across long distances, how to convert chemical energy into useful work, how to capture, bind, and transport nutrients, how to perceive light, how to create a sense of time and place, among a million other things that organisms like us take for granted. But most of all, bacterial cells solve the problem of surviving in their environment, whatever it is, no matter how inhospitable. How can you make it in a place that’s pitch black, under crushing pressure, and seemingly barren and without promise? Just ask whoever lives there. ♦