The Mariana Trench, a vast crescent-shaped scar in the western Pacific Ocean, represents the deepest known point on Earth. It stretches over 2,550 kilometers long and averages 69 kilometers wide. The deepest section, the Challenger Deep, plunges to an estimated 10,935 meters below the surface, a depth that exceeds the height of Mount Everest by more than two kilometers. For centuries, the trench remained a profound mystery. Early attempts to plumb the darkness, such as the 1960 descent of the bathyscaphe Trieste, offered the first glimpses into this unexplored frontier. Modern exploration has since revealed that this remote environment is home to surprising discoveries.
The Conditions of the Deepest Ocean
The environment of the hadal zone, the region below 6,000 meters that includes the Mariana Trench, is defined by extreme abiotic factors. The most immediate condition is the hydrostatic pressure, which at the floor of the Challenger Deep exceeds 1,000 times the atmospheric pressure at sea level. This intense pressure influences the chemical and physical properties of water, constraining life in shallower waters. Organisms must contend with temperatures that hover just a few degrees above freezing, typically between 1 and 4 degrees Celsius. This combination of cold and pressure necessitates highly specialized biological machinery.
The environment is also one of perpetual darkness, as sunlight is completely blocked out hundreds of meters above the trench floor. The lack of light means that the base of the food web cannot be sustained by photosynthesis, forcing life to rely on alternative energy sources.
Life Forms Adapted to Extreme Pressure
The organisms discovered in the Mariana Trench have evolved specific adaptations to survive the high-pressure environment. One notable find is the Mariana snailfish (Pseudoliparis swirei), the deepest-dwelling fish ever recorded, observed at depths exceeding 8,000 meters. These translucent, gelatinous fish lack the gas-filled swim bladders that would implode under the hadal pressure.
Invertebrates dominate the deepest parts of the trench, including small, shrimp-like crustaceans called amphipods. These scavengers are abundant and play a significant role in the deep-sea food chain. Research on these amphipods revealed that some possess a unique biological mechanism, using aluminum compounds to strengthen their exoskeletons. Another unique organism is the xenophyophore, a giant single-celled protozoan that can grow up to 10 centimeters long. These fragile, sponge-like creatures create intricate shells by gathering particles from the surrounding water.
At a cellular level, deep-sea life relies on specialized organic molecules known as piezolytes to maintain protein function under pressure. These compounds stabilize the proteins within the cell, preventing them from losing function. Diverse microbial communities also thrive in the sediment by processing organic matter that drifts down from the upper ocean layers.
Geological Structures and Features
The Mariana Trench is a dynamic geological system formed by the process of subduction. This occurs where the Pacific Tectonic Plate is forced beneath the smaller Mariana Plate, creating a deep, arcing depression in the seafloor. This tectonic activity results in a complex environment that includes unique rock formations and mineral deposits.
One discovery is the presence of active mud volcanoes, which erupt with a fine slurry of serpentinite mud. These features bring up rock fragments from deep within the Earth’s mantle, providing scientists with samples of metamorphic minerals. Analyzing these rock clasts has offered insights into the geological history of the subduction zone.
Further discoveries include some of the deepest known hydrothermal vents, which release superheated, chemically rich fluids that create unique ecosystems. For example, the Eifuku submarine volcano releases liquid carbon dioxide, while the Daikoku submarine volcano contains a pool of molten sulfur. These deep-sea hydrothermal vents, often referred to as “black smokers,” support chemosynthetic bacteria. These bacteria form the base of an entire food web, demonstrating that life can thrive by utilizing chemical energy in the absence of sunlight.
Evidence of Human Impact
A pervasive presence of human-made contamination has been found in the Mariana Trench. Despite its extreme depth and remoteness, the trench acts as a sink for persistent organic pollutants (POPs). These contaminants, which do not degrade naturally, find their way to the ocean floor through a process known as the “biological pump.”
Scientists have detected microplastics and other synthetic particles ingested by deep-sea amphipods collected from the Challenger Deep. In one study, over 72% of the amphipods examined contained at least one microparticle. The materials identified included synthetic fibers and plastics such as Nylon and polyethylene.
Banned industrial chemicals were also discovered in the trench fauna. High concentrations of polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) were found in the tissues of scavenging amphipods. PCBs were once used in electrical equipment and paints, while PBDEs were common flame retardants. The levels of PCBs detected were, in some cases, higher than those found in crustaceans from highly polluted rivers in China. These pollutants are transported to the deepest trenches as they attach to sinking organic matter and sediment.