Almost none of the Mariana Trench has been directly explored. Only a handful of crewed and robotic missions have ever reached the trench floor, and the areas they covered amount to tiny pinpoints across a feature that stretches roughly 2,550 kilometers. Even high-resolution sonar mapping, which gives a detailed picture of the seafloor without anyone going down, has covered only a fraction of the trench’s full extent. As of mid-2024, just over a quarter of the entire global ocean floor has been mapped at modern resolution, and the Mariana Trench, despite its fame, is no exception to that gap.
What “Explored” Actually Means at Full Ocean Depth
When people ask how much of the Mariana Trench has been explored, the answer depends heavily on what counts as exploration. There are several levels, and they differ wildly in coverage. Satellite-derived gravity data can estimate the general shape of the seafloor across virtually the entire ocean, but the resolution is coarse, on the order of a few kilometers per pixel. That is enough to tell you a trench exists and roughly how deep it is, but not enough to reveal individual geological features, volcanic structures, or the fine topography that scientists need. High-resolution multibeam sonar, conducted from ships, can map seafloor features in far greater detail, but it requires a vessel to physically transit over the area, and deep water slows the process because the sonar beam spreads with depth. Then there is direct exploration: sending instruments, cameras, or people to the bottom. That remains extraordinarily rare.
The distinction matters because most public maps of the ocean floor are built from satellite gravity models, which can give the illusion that we know more than we do. When researchers talk about the percentage of the ocean that has been “mapped,” they typically mean mapped with modern multibeam sonar at a resolution useful for science. By that standard, as of June 2024, only about 26.1% of the global ocean floor had been mapped.1Elsevier (Advances in Space Research). Seafloor topography inversion and quality assessment in complex marine regions based on SWOT_02/03 altimetry data The Mariana Trench sits among the most remote and difficult-to-reach regions on Earth, so its coverage is spotty despite the attention it receives.
How Sonar Surveys Have Mapped the Trench
The most ambitious modern mapping effort to touch the Mariana Trench was the Five Deeps Expedition, a privately funded 10-month campaign in 2018–2019 that aimed to reach the deepest point in every ocean. Over that period, the expedition mapped roughly 550,000 square kilometers of seafloor in total, using a state-of-the-art full-ocean-depth multibeam sonar system. About 61% of that coverage was entirely new ground that had never been surveyed before, and approximately 30% came from the deepest trenches and fracture zones, including the Mariana Trench.2Geoscience Data Journal. High‐resolution multibeam sonar bathymetry of the deepest place in each ocean The expedition confirmed the depth of Challenger Deep at 10,924 meters, plus or minus 15 meters, making it the deepest confirmed point on Earth.
That sounds like a lot of mapping, and it was, but spread across all five oceans. The Mariana Trench alone covers an enormous area. The trench is a crescent-shaped gash in the western Pacific, averaging about 69 kilometers wide and running for thousands of kilometers. The Five Deeps sonar swaths, while groundbreaking, still represent narrow corridors along the ship’s track rather than wall-to-wall coverage. Large sections of the trench’s inner walls, subsidiary deeps, and flanking abyssal plains remain unmapped at high resolution.
Crewed Descents to the Bottom
If we define “explored” as a human being physically present at the bottom, the number is startlingly small. The first crewed dive to Challenger Deep happened in 1960, when the bathyscaphe Trieste carried two people to a depth of roughly 10,916 meters. For over 50 years, nobody went back. In 2012, filmmaker James Cameron made a solo descent in the Deepsea Challenger submersible, spending a few hours on the bottom collecting samples and filming. Then came the dives that really changed the count: over the course of just 10 days during the Five Deeps Expedition, the submersible DSV Limiting Factor made a record five dives to the Mariana Trench floor, four to Challenger Deep and one to the Sirena Deep.3Marine Technology Society Journal. Recovery of a Lost Subsea Asset at Full Ocean Depth in the Mariana Trench (10,925 m ± 4) Using a Crewed Submersible
Since those dives, a small number of additional crewed descents have been made, some by Chinese submersibles including the Fendouzhe, which reached Challenger Deep in 2020. But even with these accomplishments, the total number of people who have visited the deepest part of the ocean remains in the single digits. For context, more people have walked on the Moon. Each crewed dive explores only the immediate area around the landing site, perhaps a few hundred meters of seafloor at most. The trench is vast, and these visits are brief, limited by battery life, life-support endurance, and the sheer time it takes to descend and ascend nearly 11 kilometers of water column.
Why the Trench Remains So Poorly Explored
The Mariana Trench sits under roughly 7 to 11 kilometers of seawater, and the pressure at the bottom exceeds 1,000 atmospheres. Building equipment that can withstand that kind of force is expensive, slow, and technically demanding. Every component of a deep submersible or remotely operated vehicle has to be engineered for conditions that would crush ordinary materials. The electronics, cameras, buoyancy systems, and sample-collection tools all require specialized housings, and failures at depth are often unrecoverable.
Even robotic landers, which are simpler than piloted submersibles, face significant logistical hurdles. A single lander deployment might take an entire day: hours to descend, hours on the bottom, and hours to ascend. The ship supporting the operation must remain on station the whole time, burning fuel and occupying expensive vessel time. Multibeam sonar mapping is faster but still slow in deep water because the sonar beam fans outward, meaning each pass of the ship only covers a strip of seafloor proportional to the water depth. At hadal depths the swath is wider than in shallower water, but the ping rate drops, so the ship must move slowly to maintain data quality.
Remoteness compounds the problem. The Mariana Trench lies far from major ports, adding transit time and cost to every expedition. Weather windows in the western Pacific are unpredictable, and operations can be delayed or cancelled by storms. These practical constraints mean that the trench gets visited only during dedicated, well-funded campaigns, not as a routine part of ocean science.
Life in the Deepest Water on Earth
Despite the limited number of visits, each expedition to the trench has returned with biological discoveries that reshape our understanding of where life can survive. One of the most striking findings involved snailfish, a family of small, gelatinous fish that turn out to thrive at pressures that would destroy most vertebrates. Researchers using baited camera landers documented two previously unknown species of snailfish in the Mariana Trench. One, informally called the “Mariana snailfish,” was observed between 6,198 and 8,076 meters and captured in traps as deep as 7,966 meters, making it the deepest-caught fish specimen with verified depth data. A second species, dubbed the “Ethereal snailfish,” was spotted as deep as 8,145 meters, the deepest any fish had been seen alive at the time of reporting.4Elsevier / Deep Sea Research Part I. Fishes of the hadal zone including new species, in situ observations and depth records of Liparidae
Beyond fish, the trench harbors dense populations of amphipods, small crustaceans that swarm around food falls and bait. These creatures have become subjects of study not only for their biology but for what they inadvertently reveal about human pollution, a topic covered further below. At the microbial level, the trench teems with bacteria and archaea that have evolved distinct genomic strategies for surviving extreme pressure. Genomic analyses of hadal microbes show that modifications to cell-surface proteins and membrane structures are central to their adaptation, with mobile genetic elements like prophages and transposons apparently driving the evolutionary changes.5PubMed Central. Genomic Characteristics and Potential Metabolic Adaptations of Hadal Trench Roseobacter and Alteromonas Bacteria Based on Single-Cell Genomics Analyses More recently, researchers building a comprehensive gene catalog of the Mariana Trench microbiome found that the vast majority of recovered bacterial and archaeal genomes, over 93%, contained genes for metabolizing D-amino acids, a biochemical flexibility that appears to be a common survival strategy in the hadal zone.6PubMed Central. D-amino acid metabolic versatility as a common adaptive strategy in the Mariana Trench microbiome
Pollution Has Already Reached the Bottom
One of the more sobering findings from trench exploration is that human-made contaminants are already there, even though almost no humans have been. Researchers examining amphipods collected from the Mariana Trench and five other deep trenches found synthetic microfibers and plastic fragments in the guts of animals at every site. In the Mariana Trench, the ingestion rate was 100%: every single amphipod examined had consumed microplastic.7PubMed Central. Microplastics and synthetic particles ingested by deep-sea amphipods in six of the deepest marine ecosystems on Earth
Sediment samples tell a similar story. Microplastic concentrations in sediment from the Challenger Deep region reached as high as about 71 items per kilogram of dry sediment.8PubMed. The ocean’s ultimate trashcan: Hadal trenches as major depositories for plastic pollution Given how slowly tiny plastic particles sink through the water column, the fact that they accumulate in such quantities at nearly 11 kilometers deep suggests that other processes are at work, likely the same downslope transport mechanisms that funnel sediment and organic carbon into the trench. Larger plastic debris has also been recovered from hadal depths. In a survey of macroplastic sampled down to 9,600 meters in Pacific trenches, industrial packaging and fishing-related materials were the most common items, primarily made of polyethylene, polypropylene, and nylon. Some pieces were only partially broken down, meaning intact trash is reaching the deep ocean and persisting.9PubMed. Journey to the deep: plastic pollution in the hadal of deep-sea trenches
What Lies Beneath the Trench Floor
Exploration of the Mariana Trench is not limited to its surface and inhabitants. Geophysicists have used ocean-bottom seismometers to study the rock structure and seismic activity deep beneath the trench, revealing details about how the Pacific Plate slides beneath the Philippine Sea Plate at one of the most active subduction zones on Earth. A seismometer network deployed near Challenger Deep between December 2016 and June 2017 detected more than 1,975 earthquakes, including an outer-rise fault cluster that extended down to about 50 kilometers below the seafloor.10Geophysical Research Letters. Deep Outer‐Rise Faults in the Southern Mariana Subduction Zone Indicated by a Machine‐Learning‐Based High‐Resolution Earthquake Catalog
These deep faults are more than a geological curiosity. As the Pacific Plate bends and cracks on its way down into the mantle, seawater penetrates into the fractures and hydrates the rock. Seismic imaging around the central Mariana Trench has shown that the zone of hydrated mantle extends roughly 24 kilometers below the boundary between the crust and mantle.11PubMed. Water input into the Mariana subduction zone estimated from ocean-bottom seismic data This means enormous volumes of water are being carried into the deep Earth, with implications for mantle chemistry, volcanism, and the long-term cycling of water between the ocean and the planet’s interior. Complementary seismic profiling across the southernmost trench, using a 349-kilometer-long line of instruments running from the subducting plate through Challenger Deep to the overriding plate, has helped refine our picture of the crustal architecture in this region.12Journal of Geophysical Research: Solid Earth. Deep Seismic Structure Across the Southernmost Mariana Trench: Implications for Arc Rifting and Plate Hydration
The Trench as a Carbon Trap
Another dimension of trench exploration involves understanding how the Mariana Trench functions as a repository for organic carbon. Sediment cores from the southern trench reveal that organic carbon has been accumulating continuously since the last ice age, at estimated rates of roughly 529 to 942 grams of carbon per square meter per thousand years. That rate is one to two orders of magnitude higher than what occurs on the flat abyssal plains surrounding the trench.13Chemical Geology. Continuous deposition of pre-aged organic carbon in the southern Mariana Trench since the last deglaciation The carbon reaching the trench floor is not freshly produced. Its radiocarbon ages indicate it is “pre-aged” material that has been sitting on shallower slopes for thousands of years before being swept downhill by gravity-driven transport.
A broader study of sediment accumulation across four hadal trench systems confirmed that this funneling effect is a general feature of trenches, not unique to the Mariana. During normal periods without major sediment avalanches, downslope focusing still steadily enriches the trench interior with organic material. When occasional mass-wasting events do occur, they can boost carbon deposition rates by up to a factor of roughly four over recent timescales.14Journal of Geophysical Research: Biogeosciences. Sediment Accumulation and Carbon Burial in Four Hadal Trench Systems The finding reframes hadal trenches from barren chasms to active participants in the global carbon cycle, locking away organic matter in locations so deep and remote that it is effectively sequestered from the atmosphere for geological timescales.
Listening to Challenger Deep
One novel approach to “exploring” the Mariana Trench has involved simply listening. In a 2015 deployment, researchers lowered a hydrophone to the bottom of Challenger Deep and recorded continuously for 24 days. The instrument sat at a depth corresponding to a pressure of over 11,161 decibars, roughly 10,830 meters down. What they heard was surprisingly noisy. In the low-frequency band associated with ship traffic, sound levels at the trench floor were comparable to those caused by moderate shipping activity at the surface, almost certainly because of persistent commercial and military vessel traffic in the western Pacific. Wind and wave noise also penetrated to the bottom, with levels sometimes equivalent to a sea state of 2, though during calm periods the soundscape dropped to near silence.15Oceanography. Ambient Sound at Challenger Deep, Mariana Trench
The result was a reminder that even the most extreme environments on Earth are connected to the surface world. Ship noise, storm activity, and biological sounds all propagate through the water column to the deepest point in the ocean. For scientists, the acoustic data also serve a practical purpose: understanding the ambient noise floor helps calibrate instruments used for seismic monitoring and biological surveys at hadal depths.
The Gap Between Fame and Knowledge
The Mariana Trench occupies a unique position in public imagination. It is the go-to reference for “the deepest place on Earth,” the setting for movies and documentaries, and the benchmark against which all ocean depths are measured. Yet this fame has not translated into proportional scientific attention. A review of marine research across the Mariana Archipelago, covering work from the early 1900s through 2018, emphasized that the region’s research history is heavily skewed toward fisheries and shallow-water ecology conducted by NOAA Fisheries and local management agencies in Guam and the Commonwealth of the Northern Mariana Islands.16NOAA Institutional Repository. An Overview of marine research in the Mariana Archipelago: 1900–2018 The deep hadal zone has been touched by only a few dedicated campaigns, most of them in the past decade.
The global initiative known as Seabed 2030, which aims to map the entire ocean floor at modern resolution by the end of this decade, could change the picture for the Mariana Trench along with the rest of the deep sea. But mapping is just one dimension. Biological surveys, sediment sampling, geochemical analyses, and long-term environmental monitoring all require repeat visits with specialized equipment. The trench remains a place where each new expedition produces firsts: first acoustic recording, first microplastic census, first microbial genome catalog. That is exciting, but it also means the baseline understanding is still being built. We are not revisiting known territory and refining details. We are still in the discovery phase for most of what lies inside the deepest feature on the planet.