The notion that the most alive places on Earth are also the deepest is subtly unnerving. Not in the same way as a coral reef, which is noisy, vibrant, and competitive, but in the same way that an abandoned building can occasionally feel alive—something is moving inside, but you’re not sure what.
The majority of ocean science essentially ends at the hadal trenches. The water column compresses into a type of permanent, pressurized darkness below 6,000 meters. However, things are erratic here. Transparent amphipods, snailfish with skin so thin that their organs are visible, and holothurians that move across sediment as though they have endless time in the universe are examples of pale, almost architectural creatures. Watching footage from robotic landers dropped into these zones makes it difficult to ignore the fact that life down here seems almost purposefully strange, as if evolution were showcasing itself in a room where no one was supposed to be.
Even though the pressure is extremely high—up to 1,100 atmospheres at Challenger Deep—it is not the only factor that makes studying the hadal world particularly challenging. The circulation is the cause. The ocean’s deep water is never still. Dense and frigid, Antarctic Bottom Water originates close to the poles and flows north along basin floors, slowly pushing through gaps and fracture zones in mid-ocean ridges while carrying sediment, oxygen, and whatever else it picks up along the way. Similar actions are taken in the opposite direction by North Atlantic Deep Water, which spills over shallow ridges between Greenland and Iceland, cascades down continental slopes, and eventually moves south as the deep limb of the Atlantic’s overturning circulation. At the end of these massive conveyor systems are the hadal trenches, which are essentially where water settles and shapes all subsequent biological communities.
The realization that the organisms residing in hadal trenches are not isolated seems to have been circling around it for decades without quite putting it out there. They are physically and chemically linked to currents that started thousands of kilometers away. Signals from the North Atlantic surface are being carried all the way to the basin floor by a section of Iceland-Scotland Overflow Water that has been tracked across the Charlie-Gibbs Fracture Zone. The timescales are astounding. According to tracer studies conducted in the 1990s, it took four to five years for intermediate water to travel from the Labrador Sea to the eastern boundary, moving at a speed of about 1.5 to 2 centimeters per second across the North Atlantic. In a strange way, the hadal creatures riding those water masses at the other end of the journey are getting old mail.
Researchers are constantly surprised by what actually resides in these areas. No one anticipated seeing the snailfish, which belong to the Liparidae family, moving in loose shoals at depths of more than 8,000 meters. At depths where both were thought to be insignificant, shoaling behavior suggests social structure, which suggests something about resource distribution and predation pressure. Amphipods are shrimp-like scavengers that can be found throughout the deep sea. They feed on organic matter that drifts down from the surface world and can reach astounding population densities in hadal trenches. They work with what can only be described as enthusiasm as the cleanup crew of an ecosystem that most people will never see. When scientists use baited landers, they have occasionally recovered equipment that has been so completely overrun by amphipods that the bait is completely gone in a matter of hours.
Whether the hadal zone should be viewed as a single, cohesive ecosystem or as a collection of distinct ones divided by thousands of kilometers of open abyssal plain is still up for debate. Because of the local topography, the chemistry of the incoming water masses, and the specific kinds of organic material that sink into them, each trench may have unique communities. The fauna of the Mariana Trench is significantly different from that of the Kermadec, which is different from that of the Atacama. It is genuinely unclear whether these differences are the result of environmental forcing, evolutionary isolation, or just sampling limitations. The research community appears to favor a combination of all three, which may be the most truthful viewpoint.
Beyond any particular discovery, the most striking aspect of the hadal world is how much of it is still dark—not figuratively, but literally unexplored, unmapped, and unobserved. More objects have been sent to the lunar surface than landers to depths of less than 8,000 meters. The creatures floating through those translucent columns of pressurized water are part of a kingdom that is still, for the time being, almost wholly their own.
