Fewer than 30 people have ever reached the bottom of the Mariana Trench, making it one of the most exclusive destinations on Earth. For over half a century after the first crewed dive in 1960, only three humans had touched the deepest seafloor. That number has climbed since 2019, but it still pales in comparison to the roughly 680 people who have traveled to space. The disparity says a lot about the unique engineering, financial, and physiological challenges of reaching a place nearly 11 kilometers beneath the ocean surface.
The First Two People and a Very Long Wait
On January 23, 1960, Swiss oceanographer Jacques Piccard and U.S. Navy Lieutenant Don Walsh squeezed into the cramped steel sphere of the bathyscaphe Trieste and descended into the Challenger Deep, the deepest known point in the Mariana Trench. The dive took nearly five hours down and more than three hours back up. They spent roughly 20 minutes on the bottom before a cracked outer window prompted them to begin their ascent. For the next 52 years, no human being followed them.
That gap was not due to lack of interest. The engineering demands of a full-ocean-depth vehicle are extreme. At the bottom of the Challenger Deep, water pressure exceeds 1,000 times atmospheric pressure at sea level. The Trieste itself was retired in 1966, and no nation or private entity built a replacement capable of reaching those depths until the 2010s. Robotic vehicles visited the trench periodically during those decades, but no person did.
James Cameron’s Solo Dive in 2012
The filmmaker and explorer James Cameron became the third person to reach the bottom of Challenger Deep on March 26, 2012, piloting a one-person vertical submarine called the Deepsea Challenger. Unlike the Trieste, which was a two-person craft with limited scientific capability, Cameron’s vehicle was designed to collect samples and footage at depth. Cameron spent several hours on the bottom and later presented initial science findings from the expedition at the American Geophysical Union’s Fall Meeting that year.1Eos, Transactions American Geophysical Union. James Cameron discusses record dive and science concerns
Cameron’s dive was a solo affair, and after it the total count of people who had ever been to the bottom of the Mariana Trench stood at exactly three. The Deepsea Challenger was not used for subsequent full-ocean-depth dives, and the sub was eventually donated to the Woods Hole Oceanographic Institution. So another gap opened, though this one would prove much shorter.
2019 and the Shift to Repeat Dives
The real change came in 2019, when American explorer Victor Vescovo and his team completed the third leg of the Five Deeps Expedition. Over just ten days, their submersible, the DSV Limiting Factor, made a record five dives to the bottom of the Mariana Trench: four to Challenger Deep and one to Sirena Deep, a separate depression in the same trench.2Marine 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 The Limiting Factor is a two-person submersible, and Vescovo brought along various scientists and crew members across those dives. This single expedition roughly doubled the total number of people who had reached the trench floor.
What made the Limiting Factor different from its predecessors was reusability. The Trieste was a one-mission wonder at full ocean depth. The Deepsea Challenger managed a single crewed dive. Vescovo’s sub, built by Triton Submarines, was designed from the start for repeated certification and descent, and it has continued operating since 2019. In subsequent years, the Limiting Factor has carried additional scientists, a handful of paying adventurers, and even Kelly Walsh, son of Don Walsh, to the bottom of the Challenger Deep. Each of those passengers added to the cumulative tally.
China’s Fendouzhe and the Growing List
In November 2020, China’s Fendouzhe (meaning “Striver”) human-occupied vehicle reached the bottom of the Challenger Deep, making China only the third nation to send people to the deepest point in the ocean. Fendouzhe carried a crew of three on its deepest dive and completed multiple descents during its expedition. The vehicle’s acoustic system, which handles communication, navigation, and imaging in an environment where radio signals cannot penetrate, represented a significant engineering effort in its own right.3PubMed Central. The Acoustic System of the Fendouzhe HOV
Between the Limiting Factor’s ongoing dives and the Fendouzhe missions, the total number of people who have personally visited the bottom of the Mariana Trench has been climbing steadily. As of late 2024, that number sits somewhere in the high twenties, depending on how you count certain unpublicized dives. More people have walked on the Moon than visited Challenger Deep for most of the history of deep-sea exploration, and it was only in the early 2020s that the trench tally finally surpassed the twelve moonwalkers.
Why the Number Is Still So Small
The pressure at full ocean depth is the central engineering problem. At nearly 11 kilometers down, every square centimeter of a vehicle’s hull bears roughly 1,100 kilograms of force. Submersible pressure hulls are typically built as spheres or near-spheres from high-strength steel or titanium, because a sphere distributes compressive force more evenly than any other shape.4International Journal of Pressure Vessels and Piping. A study of diving depth on deep-diving submersible vehicles Material choice, wall thickness, and the tiniest manufacturing imperfections all determine whether a hull holds or buckles. Composite materials have been studied as alternatives to metal, with carbon-epoxy and other layups offering potential weight savings, but the design optimization is complex and the consequences of getting it wrong are immediate and fatal.5PubMed Central. Design Optimization and Non-Linear Buckling Analysis of Spherical Composite Submersible Pressure Hull
Beyond the crew sphere, every other component of a deep submersible must also survive the pressure. Buoyancy materials, typically syntactic foams made of tiny hollow glass or ceramic spheres embedded in a resin matrix, allow the vehicle to float back to the surface after dropping its ballast. These foams must be both light enough to provide buoyancy and strong enough not to collapse under hydrostatic pressure, a tradeoff that becomes increasingly difficult to strike at greater depths.6European Journal of Mechanics – A/Solids. The failure behavior of syntactic foams as buoyancy materials for deepsea applications Cameras, lights, manipulator arms, thrusters, and acoustic instruments all need their own pressure housings or must be designed as pressure-tolerant from the start.
Then there is cost. Building a full-ocean-depth submersible takes years and tens of millions of dollars. Operating it requires a support ship, a trained crew, favorable weather windows, and transit time to one of the most remote stretches of the Pacific Ocean. The Mariana Trench lies roughly 200 kilometers southwest of Guam, in open ocean far from commercial shipping lanes. A single expedition can cost millions, and the pool of organizations willing and able to fund such trips is tiny.
How Deep Is the Bottom, Exactly?
Pinning down the precise depth of the Challenger Deep has been a project in itself. Different survey methods have produced slightly different numbers over the decades. The most recent and most rigorous measurement comes from submersible transects using in-situ pressure sensors, corrected for water-column properties, atmospheric pressure, gravity anomalies, and tidal effects. That work placed the deepest observed seafloor at 10,935 meters below mean sea level, with an uncertainty of about 6 meters.7Deep Sea Research Part I: Oceanographic Research Papers. Revised depth of the Challenger Deep from submersible transects; including a general method for precise, pressure-derived depths in the ocean
You will sometimes see the depth quoted as 10,994 meters, 10,984 meters, or 10,916 meters in different sources. The variation comes from differences in survey technology, how pressure is converted to depth, and which specific spot within the Challenger Deep was measured. The Challenger Deep itself is a crescent-shaped valley about 11 kilometers long, and its floor is not perfectly flat, so the deepest point shifts slightly depending on where you sample. None of this changes the practical reality: the bottom is nearly 11 kilometers down, give or take a few meters that matter to cartographers but not to the people building subs.
What Lives at the Bottom
One of the reasons people go to the trouble of visiting the trench in person is the life they find there. The Mariana Trench’s hadal zone hosts organisms that have evolved remarkable adaptations to crushing pressure, near-freezing temperatures, and total darkness. The Mariana hadal snailfish, a small translucent fish found at depths exceeding 8,000 meters, is among the best-studied examples. Genomic analysis of the snailfish revealed several unusual features: a mutation that appears to prevent its cartilage from fully hardening into bone, expanded gene families that help maintain cell membrane fluidity under pressure, and changes to enzymes that stabilize proteins in high-pressure conditions.8Nature Ecology & Evolution. Morphology and genome of a snailfish from the Mariana Trench provide insights into deep-sea adaptation
The snailfish’s genome carries roughly three times the normal number of copies of a gene involved in making DHA, a polyunsaturated fatty acid that keeps cell membranes flexible. At extreme pressure, ordinary cell membranes would stiffen and lose function, so the additional DHA production essentially counteracts the physical compression. The fish also has expanded gene families for ion and solute transport, consistent with the need to keep fluids moving through tissues despite pressure-induced resistance.8Nature Ecology & Evolution. Morphology and genome of a snailfish from the Mariana Trench provide insights into deep-sea adaptation These are not minor tweaks. The genome of this fish reflects thousands of generations of selection for survival in conditions that would kill virtually any other vertebrate.
Below the depth range where fish can survive, the hadal zone still teems with microbial life, amphipods (small crustacean scavengers), and other invertebrates. Researchers have retrieved samples from the trench floor showing thriving bacterial communities, including microbes that appear to derive energy from chemical reactions rather than sunlight-driven food chains. Much of this biology remains poorly cataloged precisely because so few people and vehicles have visited.
The Human Body at Depth
Inside a full-ocean-depth submersible, the crew breathes air at roughly normal atmospheric pressure, so the occupants are not directly exposed to the crushing water pressure outside. The danger is entirely structural: if the hull fails, the occupants would have no warning and no chance of survival. Life support in these vehicles must manage oxygen supply, carbon dioxide removal, temperature control, and humidity in a sealed space that is typically about the size of a large closet. The occupants sit with their knees drawn up for hours at a time during descent and ascent.
The psychological dimension is real, too. Spending hours in a tiny sealed sphere, knowing that the nearest rescue is kilometers of water away, creates a stress environment that has some parallels to spaceflight. Research on isolated, confined, and extreme environments, from undersea habitats to the International Space Station, has shown measurable physiological and psychological effects even at far less dramatic depths.9Therapeutic Sciences: Emerging Research in Medical Gases and Regenerative Biology – The International Hyperbarics Association Journal. Physical, physiological and psychological effects of 100 days in an undersea habitat with a combination of dry and wet diving: Monitoring Project NEPTUNE 100 Full-ocean-depth dives are shorter than extended habitat stays, but the stakes are considerably higher: there is no option to abort mid-dive and swim to the surface.
Interestingly, the people who have made these dives report a range of experiences. Cameron described the bottom of the Challenger Deep as a “desolate, lunar-like” landscape. Vescovo has spoken about the profound silence. Several participants in the Chinese Fendouzhe missions emphasized the emotional weight of seeing the deepest point on Earth firsthand, an experience no camera feed can replicate.
Crewed Dives Versus Robotic Exploration
A reasonable question is why anyone needs to go in person at all. Unmanned vehicles, both remotely operated vehicles tethered to a surface ship and autonomous underwater vehicles that operate independently, have been visiting the Mariana Trench since the 1990s. They can stay longer, carry more instruments, and do not put human lives at risk. The Japanese robotic vehicle Kaiko reached Challenger Deep in 1995 and collected sediment samples. More recently, several Chinese and American robotic platforms have mapped and sampled the trench floor extensively.
The case for crewed dives rests partly on capability and partly on something harder to quantify. A human pilot can make real-time decisions about where to sample, how to approach an unexpected feature, and what to investigate further. Robotic vehicles, even sophisticated ones, operate within the limits of their programming and the bandwidth of their communication link. At full ocean depth, acoustic communication is slow and limited, which constrains how much guidance an operator on the surface can provide.3PubMed Central. The Acoustic System of the Fendouzhe HOV A trained scientist in a submersible can notice something unexpected and redirect the dive on the spot, something that remains difficult for remotely operated systems and impossible for pre-programmed autonomous ones.
There is also the advocacy angle. Cameron’s dive in 2012 generated global media attention and public interest in deep-ocean science. Vescovo’s Five Deeps Expedition did the same. Robotic dives, however scientifically productive, rarely make headlines. The presence of a human being at the bottom of the ocean turns an engineering achievement into a story people pay attention to, and that attention translates into funding, political will, and public support for ocean research.
Commercial Trips and the Future Count
Since 2020, the Limiting Factor (later joined by a sister vehicle) has been offered for expedition-style charters, and a small number of private individuals have paid for the experience of descending to the Challenger Deep. These are not luxury tourism in the conventional sense. Passengers undergo safety briefings, squeeze into a titanium sphere barely large enough for two, and endure a roughly four-hour descent in cold, cramped conditions. The price tag has been reported in the range of several hundred thousand dollars per seat.
This commercial avenue is the primary reason the total headcount keeps climbing. Each successful charter dive adds one or two names to the list. If operations continue at their current pace, the number of people who have visited the bottom of the Mariana Trench could pass 40 or 50 within the next several years. Whether it ever becomes routine is another question. The logistical overhead of getting a support ship and submersible to the western Pacific, the weather dependency, and the inherent risk of operating at full ocean depth all impose hard limits on how frequently dives can happen.
China has signaled interest in building additional full-ocean-depth crewed vehicles, and several private ventures have discussed plans for new deep-submersibles. If even one or two more operational vehicles enter service, the rate of crewed dives could increase significantly. For now, though, the Limiting Factor remains the only repeatedly certified full-ocean-depth submersible in regular operation, and most of the growth in the headcount depends on its continued use.
The Trench Beyond Challenger Deep
It is worth noting that the Mariana Trench is not just the Challenger Deep. The trench stretches roughly 2,550 kilometers across the western Pacific, and it contains multiple depressions deeper than 10,000 meters. The Sirena Deep, which Vescovo visited in 2019, reaches about 10,714 meters. Other named features include the HMRG Deep and the Western Pool. Most crewed dives have focused on the Challenger Deep because it is the single deepest point, but the scientific interest extends across the entire trench system, and some of the most interesting biology and geology may lie in less-visited sections.
Mapping the trench at high resolution remains incomplete. While multibeam sonar from surface ships has produced broad bathymetric charts, the fine-scale topography of the trench floor, the kind that reveals individual rock outcrops, hydrothermal seeps, or biological communities, requires close-range surveys by submersibles or deep-towed instruments. Each crewed dive adds a small patch of detailed observation to what is still a mostly blank map. The trench is, in a very real sense, less explored than the surface of Mars.