Fewer people have visited the hadal zone than have orbited in space, but the exact count depends on where you draw the line. If you’re asking about the very deepest reaches of the ocean, the bottom of the Mariana Trench’s Challenger Deep, the total stood at roughly two dozen people through the early 2020s and has continued to climb since then. If you’re counting the broader hadal zone, which begins at around 6,000 meters below the surface, the number is somewhat larger thanks to research submersibles that operate in the upper hadal range. Either way, the figure is strikingly small, and the story of how it grew from two people in 1960 to a few dozen today says a lot about both the difficulty of deep-ocean exploration and the recent shift in who is doing it.
The First Two People and a Very Long Wait
On January 23, 1960, U.S. Navy Lieutenant Don Walsh and Swiss oceanographer Jacques Piccard squeezed into the cramped gondola of the bathyscaphe Trieste and descended to approximately 10,800 meters in the Challenger Deep, the deepest known point on Earth’s seafloor.1Journal of Ocean Engineering and Technology. Operational Strategies for ROVs in Underwater Search and Rescue During Large-Scale Marine Disasters They spent about twenty minutes on the bottom before beginning their ascent. At the time, this seemed like the opening chapter of an era of deep-ocean human exploration. It was not. More than half a century would pass before another person followed them down.
During that long gap, the hadal zone was not completely ignored. Unmanned vehicles made important visits. Japan’s remotely operated vehicle Kaiko reached 10,920 meters in the Mariana Trench in 1996 and collected biological specimens from the seafloor.1Journal of Ocean Engineering and Technology. Operational Strategies for ROVs in Underwater Search and Rescue During Large-Scale Marine Disasters Instruments were also lowered on cables to measure temperature, salinity, oxygen, and nutrient levels at depths exceeding 10,900 meters.2Deep Sea Research. Measurements of water characteristics at depths greater than 10 km in the Marianas Trench But none of this involved a human being actually descending into the hadal zone. For decades, Walsh and Piccard remained the only two people who had ever been there.
James Cameron Breaks the Silence
On March 26, 2012, filmmaker and explorer James Cameron piloted the Deepsea Challenger solo to the bottom of the Challenger Deep, becoming the third person in history to reach full ocean depth.1Journal of Ocean Engineering and Technology. Operational Strategies for ROVs in Underwater Search and Rescue During Large-Scale Marine Disasters His submersible was unlike anything previously built for deep-sea work. Rather than the traditional horizontal design, the Deepsea Challenger stood upright, a 7.3-meter-tall vertical structure made largely of a patented syntactic foam that served as both flotation and structural core. Cameron sat in a steel sphere just 1.09 meters in diameter with walls 6.35 centimeters thick, attached to the bottom of the foam beam.3Deep-Sea Research Part I: Oceanographic Research Papers. Submersible- and lander-observed community patterns in the Mariana and New Britain trenches: Influence of productivity and depth on epibenthic and scavenging communities The vertical design maximized descent and ascent speed, giving Cameron more time on the seafloor, and the sub carried push-core samplers, a suction sampler, and a hydraulic manipulator arm for scientific work.
Cameron’s dive was historic, but it was still a one-person mission in a vehicle that made only a single dive to full depth. The total number of people who had reached the deepest point in the ocean was now three, and the broader hadal count had barely budged. That was about to change dramatically.
The Limiting Factor Opens the Door
The real inflection point came seven years later with the DSV Limiting Factor, a new two-person submersible built for repeated deep dives. Beginning in 2018, American explorer Victor Vescovo used the Limiting Factor to complete the Five Deeps Expedition, diving to the deepest point in each of the world’s five oceans.4Marine Technology Society Journal. The Five Deeps Expedition and an Update of Full Ocean Depth Exploration and Explorers The expedition included reaching the bottom of the Mariana Trench in 2019, making Vescovo the fourth person to visit Challenger Deep’s deepest point.
What made the Limiting Factor genuinely transformative, though, was that it kept diving. Unlike the Trieste, which was retired after limited use, and unlike the Deepsea Challenger, which made a single full-depth dive, the Limiting Factor was certified for repeated operations and carried a second seat. Over subsequent years, Vescovo brought a series of passengers to the bottom of the Mariana Trench and other hadal trenches. Scientists, engineers, a former astronaut, and even the son of Don Walsh all made the trip. The Five Deeps Expedition produced a detailed collation of all significantly deep dives and the crew members who participated.4Marine Technology Society Journal. The Five Deeps Expedition and an Update of Full Ocean Depth Exploration and Explorers By 2020, the number of people who had reached full ocean depth had already surpassed the twelve humans who walked on the Moon.
China and the Growing Count
While the Limiting Factor was racking up dives in the Western Pacific, China entered the full-ocean-depth club. In November 2020, the crewed submersible Fendouzhe (translated roughly as “Striver”) reached the bottom of the Challenger Deep, carrying three occupants. China had previously operated the Jiaolong submersible, which could reach about 7,000 meters, well within the hadal zone but far short of full ocean depth. Fendouzhe, rated for 10,000-plus meters, marked a significant step. Subsequent dives added more Chinese researchers to the roster of people who have visited the deepest waters on Earth.
By the early 2020s, the total number of people who had been to the bottom of the Challenger Deep stood somewhere in the range of 25 to 30, depending on how you count repeat visitors and which reporting you consult. That number continues to grow as both the Limiting Factor and Fendouzhe remain operational, and as new submersibles come into service. The pace is picking up: the first five decades of hadal exploration added three people, while the most recent five years have added more than twenty.
The Hadal Zone Is Bigger Than Challenger Deep
Most public attention focuses on the Mariana Trench, but the hadal zone is not a single spot. It encompasses every ocean depth below about 6,000 meters, and that terrain is scattered across roughly 33 trenches and troughs around the world, primarily ringing the Pacific Ocean. These include the Tonga Trench, the Kermadec Trench, the Philippine Trench, the Japan Trench, the Kuril-Kamchatka Trench, the Puerto Rico Trench in the Atlantic, and the South Sandwich Trench in the Southern Ocean, among others.
When you account for research submersibles that operate in the upper hadal range, the count of people who have entered the hadal zone broadens. Japan’s Shinkai 6500, as the name implies, can dive to 6,500 meters. Over its decades of operation, it has carried dozens of scientists and pilots to depths just below the hadal threshold. China’s Jiaolong operated to 7,000 meters. France’s Nautile reached 6,000 meters. Russia’s Mir submersibles went to similar depths. These vehicles have logged hundreds of dives collectively, and while not all of those dives reached hadal depth, many did.
If you define the question strictly as “who has reached full ocean depth,” the answer is a few dozen people. If you define it as “who has physically descended below 6,000 meters,” the answer is likely a few hundred, though a precise count is hard to pin down because dive logs from older research programs are not always published in compiled form. Either figure is vanishingly small compared to the roughly 600 people who have been to space.
Why the Number Stays So Small
The engineering problem is extreme. At the bottom of the Mariana Trench, the pressure is roughly 1,100 times what you feel at the ocean surface, about 1,100 bar. Everything a submersible is made of must either resist that force or be designed to compress without failing. The most critical component is the pressure hull, the sphere that keeps the human occupants alive. Current design philosophy relies on increasing shell thickness to withstand greater depths, but thicker walls mean more weight and a larger stress difference between the inner and outer surfaces of the hull.5Ocean Engineering. Strength optimization design of spherical hulls for deep-sea submersibles: A hydraulic autofrettage approach in external pressure vessels Getting that balance right, strong enough not to collapse, light enough to actually float back up, is one of the central challenges of deep submersible design.
Cost is another barrier. Building and certifying a full-ocean-depth submersible runs into the hundreds of millions of dollars. Operating one requires a dedicated surface ship, a specialized launch and recovery crew, and extensive pre-dive checks. Unlike spaceflight, which has attracted massive government and now commercial investment, deep-ocean exploration has historically operated on shoestring budgets. Most of the recent progress has been driven by private wealth: Cameron self-funded much of the Deepsea Challenger project, and Vescovo funded the Limiting Factor and Five Deeps Expedition privately.
There is also a practical question of what you gain by putting a human inside the vehicle. Remotely operated vehicles and autonomous underwater vehicles can reach hadal depths at far lower cost and risk. Comparisons of surveys conducted by autonomous vehicles and crewed submersibles show that the two platforms have complementary strengths: autonomous vehicles tend to record higher species counts across open terrain, while crewed submersibles are better at spotting animals tucked into complex structures.6Marine Environmental Research. Comparing AUV and HOV imagery for deep-sea fish biodiversity monitoring: insights from Pioneer Bank, in the Northwestern Hawaiian Islands This complementarity makes a case for continued crewed dives, but it also means that many scientific objectives can be met without sending anyone down in person.
What Hadal Visitors Actually Find
The hadal zone is not a barren wasteland. Life thrives there, though it looks nothing like what you would see at shallower depths. Amphipods, small crustaceans that resemble translucent shrimp, are the most abundant visible animals and serve as scavengers on the trench floor. Sea cucumbers, polychaete worms, and foraminifera populate the sediment. And there are fish: the Mariana snailfish, Pseudoliparis swirei, holds the record for the deepest-living fish ever observed, found at depths exceeding 8,000 meters in the Mariana Trench.
The adaptations that allow hadal animals to survive are striking. The Mariana snailfish, for example, has a skeleton that barely calcifies. Genomic work has identified a frameshift mutation in the bone Gla protein gene that appears to cause early termination of cartilage hardening, leaving the fish with a soft, gelatinous body that handles pressure better than rigid bone would.7PubMed. Morphology and genome of a snailfish from the Mariana Trench provide insights into deep-sea adaptation At the molecular level, these fish accumulate trimethylamine N-oxide, or TMAO, a small molecule that protects proteins from being crushed out of shape by pressure. The gene responsible for producing TMAO appears to have been positively selected in the Mariana snailfish and carries extra regulatory sequences that may boost its activity.8Nature Ecology & Evolution. Morphology and genome of a snailfish from the Mariana Trench provide insights into deep-sea adaptation
Cell membranes face their own pressure challenge. Under extreme compression, membranes can stiffen to the point where they stop functioning. The Mariana snailfish and other deep-sea species counter this by adjusting the ratio of fats in their membranes, increasing unsaturated fatty acids and lowering cholesterol to keep membranes fluid.9Water Biology and Security. Lipidome and proteome analyses provide insights into Mariana Trench Snailfish (Pseudoliparis swirei) adaptation to the hadal zone Among the deep-sea fish studied, the Mariana snailfish had the lowest cholesterol percentage in its liver tissue and the highest proportion of unsaturated lipids, a biochemical profile fine-tuned for life at crushing depth.
Human Pollution Got There First
One of the more sobering findings from hadal exploration is that human-made contamination preceded human visitors by a wide margin. Researchers examining amphipods collected from six of the deepest trenches on Earth found synthetic microparticles, mostly fibers, in the guts of animals at every single site. The ingestion rate ranged from 50 percent of amphipods at the New Hebrides Trench to 100 percent at the Mariana Trench. Across all trenches, about 72 percent of the 90 individual amphipods examined contained at least one microfiber or fragment.10PubMed Central. Microplastics and synthetic particles ingested by deep-sea amphipods in six of the deepest marine ecosystems on Earth
This means that by the time Victor Vescovo’s submersible reached the floor of the Mariana Trench in 2019, plastic was already a resident. The finding undercuts any notion that the hadal zone is too remote to be affected by surface activity. Microplastics sink, are carried by currents, and accumulate in trenches, which act as sediment traps. The animals living there ingest what arrives. Whether this contamination has measurable effects on hadal ecosystems remains an open question, but its presence at 100 percent frequency in Mariana Trench amphipods is hard to dismiss as trivial.
Comparing Deep-Ocean and Space Exploration Numbers
The comparison with spaceflight is worth sitting with for a moment. As of the mid-2020s, more than 600 people have traveled to space. Twelve walked on the Moon. The International Space Station alone has hosted over 270 individuals. Meanwhile, the number of people who have physically entered the hadal zone, even using the broadest reasonable definition, likely falls somewhere between one hundred and a few hundred. At the deepest point, it is a few dozen.
This asymmetry is not because the ocean is less interesting or less important than space. It is partly a function of funding priorities, partly a function of the fact that robots can do much of the scientific work in the deep ocean at lower cost, and partly a function of public imagination. Space exploration has had dedicated national agencies, geopolitical competition, and cultural mythology driving investment for more than six decades. Deep-ocean exploration has had no equivalent sustained push. The trenches do not have a NASA.
That may be slowly changing. With two proven full-ocean-depth submersibles now in regular operation, multiple nations building new deep-diving vehicles, and growing scientific interest in hadal ecosystems, the rate of crewed hadal dives is higher now than at any point in history. The number of hadal visitors in the next decade could easily exceed the total from the previous sixty years combined. But for now, the people who have seen the deepest parts of the ocean with their own eyes remain an extraordinarily small group, closer in number to the people who have summited K2 in winter than to any broader category of explorers.