The deep sea is a world of extremes. So far in this series, we’ve taken a look at the
ways in which animals have adapted to survive in the cold, dark depths. We’ve explored
the ecosystems of this unforgiving realm, and seen how communities of life cling to
whatever source of energy they may find, from vents of superheated water, to the carrion
of sunken whales. But now it’s time to dive even deeper, and discover a place where life
is pushed to its absolute limits. For at the bottom of the Pacific Ocean, the sea floor
widens into a gaping abyss of obscurities. Fish that seem to lack faces, sprawling fields
of bacteria, and giants that resemble life from another planet. Let’s take a closer
look at the hidden world of the Mariana Trench.
The infamous Mariana Trench sits like a crescent-shaped dent in the floor of the Pacific. A 2,550
km long, 69 km wide fracture that plummets down into a pure black void. At the bottom,
it hosts the deepest known location on Earth. The Challenger Deep, 11,033 metres or 36,200
feet beneath the waves. The trench itself is but one part of a global network of deep
scars that cut across the ocean floor. Features that formed from a process called subduction.
In the case of the Mariana Trench, the western edge of the Pacific Plate was thrust beneath
the smaller Mariana Plate to the west, creating the deep fracture. Molten material then rose
through volcanoes near the trench, building the nearby Mariana Islands.
At its deepest point, the Mariana Trench dips down into a little-explored zone of the ocean.
The Hadal zone, named after the realm Hades, the underworld of Greek Mythology. A suitable
title for a place where the conditions of pure darkness, acidic freezing water, scarce
food, and immense pressure create a challenging environment for creatures to survive in. For
much of history, it was believed to be a dead zone, void of any life at all. An impossible
frontier and an empty void of perils that could never be reached by any human. But in
the 19th Century, this was all about to change.
The Mariana’s depths were first plumbed in 1875 when the crew aboard the H.M.S Challenger
cast a weighted sounding rope over the side of the vessel and found they needed more rope.
They had not expected there to be a location so deep, but news its discovery caught the
eye of the ambitious. Knowing it existed simply wasn’t enough, and a few dared to venture
to the bottom.
In 1960, 85 years after the Challenger Deep was discovered on that pioneering voyage,
two men set out to reach the bottom. Jacques Piccard and Navy Lt. Don Walsh, sheltered
only by a cramped bathyscape submersible called the Trieste. Their 5-hour descent was fraught
with challenges. The water pressure near the bottom was nearly a thousand times greater
than atmospheric pressure at sea level. During the journey, this caused the viewing window
to crack, limiting their time spent on the sea floor to only 20 minutes. Even in such
a short amount of time, what they saw would shock the scientific community. Life. Pale
shrimp and flounder-like fish, along with what they described as a dark brown diatomaceous
ooze that covered the sea floor.
Piccard described this moment with excitement in a book about the voyage. “Here, in an
instant, was the answer that biologists had asked for the decades. Could life exist in
the greatest depths of the ocean? It could!”
When James Cameron followed in the Trieste’s footsteps onboard the Deepsea Challenger in
2012, he too saw the sprawling microbial mats — bizarre-looking, filamentous clumps of
microorganisms living off chemicals from altered rocks 10,912 meters (35,803 feet) down in
a sunless world. It is these bacteria that support more complex creatures, for without
sunlight, larger animals must instead rely on the energy produced by bacteria undergoing
chemosynthesis. The deep sea equivalent to photosynthesis.
The footage you’re seeing now was taken by an unmanned Japanese submersible called
Kaiko in 1996. Having reached a depth of 10,897 metres, it marked the deepest dive for an
unmanned submersible at the time. Its goal was to sample bacteria from the matts that
Piccard and Walsh had observed nearly 40 years earlier. They found that a number of these
bacterial species appeared to be obligately barophilic, meaning they thrived under high
environmental pressures, proving that our idea that life could only exist in more moderate
conditions was flawed. But in 1998, Kaiko returned to the Challenger Deep and stumbled
upon more complex life. Hirondellea gigas, a gigantic amphipod species.
This discovery posed a bit of a mystery. The extreme pressures of the deep sea cause the
calcium carbonate that makes up the shells of amphipods and many marine animals to dissolve
more readily in water, leaving their soft bodies vulnerable. As such, amphipods are
not usually found below about 5,000 m (16,400 feet). And yet here in Kaiko’s lights was
a giant amphipod, retaining even its tough exoskeleton. It’s since been found that
they protect their shells using a form of aluminium armour, using chemicals in their
gut to extract aluminium ions from the seafloor mud while they forage for food.
In their role as detrivores, these amphipods occupy a key role in the ecosystem. They act
as a clean-up crew, possessing enzymes that are able to digest even wood.
In more recent times, remote submersibles have caught yet more oddities of the Mariana
Trench in their headlights. Among the most abundant inhabitants are the holothurians.
Sea cucumbers, like the remarkable sea pig with its ring of feeding tentacles that it
uses to sift through the mud and grab onto food. Some scientists believe Piccard’s
fish was in fact a sea cucumber, for it is thought that fish are unable to survive where
the pressure is so great that it would dissolve the bones of any vertebrates. The deepest
known fish thrives at depths of 8,000 meters (26,200 feet), still 2 km above the Challenger
Deep. The Mariana Snailfish, discovered in 2014, yet given the scientific name Pseudoliparis
swirei to commemorate sub-lietunent Herbert Swire from the HMS Challenger.
But snailfish and amphipods are not the only oddballs found in the Trench. Gigantic xenophyophores
grow to be 20 cm in diameter, yet consist of only a single cell. Predatory tunicates
called sea squirts anchor their bodies to the sides of canyons in wait of passing prey,
while deep sea hatchetfish use bioluminescence to blend in with their surroundings. While
only glimpses of these otherworldly organisms have been recorded, their abundance goes to
show that life will always find a way to survive, even when faced with the intense challenges
of living at the heart of the ocean’s underworld.
Despite all the expeditions and footage that’s been gathered, we have still only just begun
to dip our toes into the hidden world of the Mariana Trench. It is likely that many new
species await discovery, and will help us piece together the puzzle of how animals can
survive such extremities. Researching the Mariana’s microorganisms could lead to vital
breakthroughs in biomedicine and biotechnology, and shed light on the story of life’s emergence
on planet Earth. But even the furthest depths are within reach of human-driven destruction.
Already has plastic been found in a place we scarcely understand. Who knows what vital
discoveries might vanish before we come to understand their importance.
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