A popular claim repeated in documentaries for decades holds that more people have walked on the moon than have visited the bottom of the ocean. Researchers who study deep-sea exploration say this comparison is not accurate.
Only 12 people have set foot on the moon, and 24 traveled there in total. By early 2023, researchers tracking crewed descents estimated that at least 27 people, possibly more than 40, had reached the Challenger Deep, the deepest point of the Mariana Trench. The first crewed descent took place in January 1960, when Jacques Piccard and Don Walsh piloted the bathyscape Trieste to the bottom and stayed for about 20 minutes. The dive nearly ended in failure when a viewing window cracked under the pressure, though the vessel held together long enough to complete the journey. The crew reported seeing what looked like a flatfish on the seafloor, settling a long-standing question about whether life could exist at the deepest point in the ocean.
No one returned for over 50 years, until filmmaker James Cameron piloted a solo submersible to a depth of roughly 10,908 meters in March 2012. After that, the pace of descents increased. Explorer Victor Vescovo made 15 separate dives to the Challenger Deep between April 2019 and July 2022, bringing a rotating group of passengers that included former NASA astronaut Kathy Sullivan, who became the first person to have both walked in space and visited the deepest point on Earth. The same program, known as the Five Deeps expedition, also made Vescovo the first person to reach the deepest point of all five of the world’s oceans within a single mission. China’s Fendouzhe submersible reached the bottom in 2020 as well.
Each descent remains an extraordinary undertaking. A single trip requires a purpose-built pressure hull, months of preparation, and a support ship stationed hundreds of kilometers from the nearest port. Between the Trieste dive and Cameron’s descent, the total number of visitors stayed in single digits for more than fifty years. The comparison between space and the ocean floor is not simply about difficulty. Both places are extremely hard to reach, but humans have now visited the trench bottom slightly more often than they have walked on the moon.
The Mariana Trench was formed by a process called subduction, where the Pacific Plate, older and denser than its neighbor, slides beneath the smaller Mariana Plate at a rate of roughly six centimeters per year. That process has continued for more than 50 million years. The trench sits within the Izu-Bonin-Mariana Subduction Zone, which stretches north from Guam toward Japan. As the sinking plate melts, the resulting rock rises to the surface further inland and built the volcanic islands of the Mariana chain.
One unusual feature of this region is a set of serpentinite mud volcanoes located just west of the trench. Water trapped in the subducting plate reacts chemically with minerals under intense heat and pressure, producing a soft mud that pushes back up through cracks in the seafloor. These formations sometimes carry pieces of ancient subducted seamounts and fossil-bearing limestone. Researchers studying these mud volcanoes have found microbial communities that survive on chemical energy rather than sunlight, an environment that has drawn attention from scientists studying how life might have first started on Earth.
The physical conditions at the trench floor are severe. The Challenger Deep reaches a maximum depth of roughly 10,935 meters, though measurements have ranged from about 10,900 to just under 11,000 meters depending on the survey method. If Mount Everest were dropped into the trench, its peak would still sit more than two kilometers underwater. Water pressure at that depth exceeds a thousand times the pressure at sea level. Seawater itself gets compressed by nearly five percent. Temperatures stay just above freezing, and no sunlight reaches anywhere close to this depth. An international mapping effort called Seabed 2030 has only charted about 27 percent of the ocean floor to modern standards as of 2025, up from six percent when the effort began in 2017.
The deep-sea creatures that inspired historical legends do not actually live at these depths. The giant squid, thought to be the basis for the Kraken myth, rarely goes below about 1,000 meters. Females can reach roughly 13 meters measured to the tips of their feeding tentacles. Nobody photographed a live one until 2004, and video of a live adult in open water did not exist until 2012. A heavier relative, the colossal squid, lives in Antarctic waters and can weigh close to 500 kilograms, with the largest examined beaks suggesting some individuals may reach 700 kilograms. A remotely operated vehicle filmed a live juvenile colossal squid for the first time in April 2025 near the South Sandwich Islands. Sperm whales, which hunt these squid, dive to about 2,000 meters on their most extreme feeding dives, which is still less than a fifth of the way down to the trench floor.
Life at the trench bottom is small and built around surviving extreme conditions. In 2014, cameras lowered into the Mariana Trench recorded a pale, soft-bodied fish at a depth deeper than any living fish had been filmed before. Researchers formally described it in 2017 as Pseudoliparis swirei, now known as the Mariana snailfish. It has no scales and a soft, jelly-like body that works with the surrounding pressure rather than resisting it. Deep-sea fish carry a compound called trimethylamine oxide that helps keep proteins stable under crushing pressure, and the deeper a fish lives, the more of this compound its tissue tends to carry.
Another resident is Alicella gigantea, a supergiant amphipod that grows past 30 centimeters, 20 to 30 times larger than its relatives. It has been recorded at depths from about 3,890 meters to nearly 8,931 meters. Genetic studies suggest a single global population of this species may stretch across more than half the world’s oceans. The strange cause of its size is debated, though leading theories point to deep-sea gigantism, where the cold and pressure may slow metabolism enough to let cells grow larger and animals live longer.
Xenophyophores are single-celled organisms roughly the size of a mango and hold the record for the largest single cell known to science. A Scripps Institution of Oceanography expedition documented them living at depths up to 10,641 meters in 2011. These giant cells host smaller organisms inside their structure and concentrate heavy metals like lead, uranium, and mercury from the surrounding water.
The more concerning discovery at the trench floor involves human pollution. In 2017, researchers led by Alan Jamieson at Newcastle University published a study in Nature Ecology and Evolution examining amphipods pulled from the Mariana and Kermadec Trenches, which sit roughly 7,000 kilometers apart. Inside the fatty tissue of these creatures, they found high levels of persistent organic pollutants, specifically polychlorinated biphenyls and polybrominated diphenyl ethers, chemicals used for decades as electrical insulators and flame retardants. PCBs were banned worldwide starting in the late 1970s due to links with cancer, yet the most contaminated samples from the Mariana Trench carried PCB levels roughly 50 times higher than crabs studied in one of the most polluted rivers in China. Jamieson said publicly that the results showed the deep ocean was not as remote and pristine as assumed.
Researchers estimate that roughly 1.3 million tons of PCBs were manufactured worldwide between the 1930s and the 1970s. About 65 percent of that total is believed to still sit inside landfills or electrical equipment. The remaining 35 percent was released into the environment and eventually worked its way into rivers, coastlines, and the open ocean. Once a pollutant sinks deep enough, the trenches function as a final resting place. Victor Vescovo has also reported seeing plastic bags and candy wrappers resting on the trench floor during his dives, and other expeditions have documented beer cans and other discarded items.
Almost every food chain on Earth’s surface traces back to sunlight. None of that applies at the bottom of the Mariana Trench. Most of the food supply down there comes from marine snow, tiny scraps of dead organic material drifting down from the surface over weeks or months. Active mud volcanoes and vents in the region release liquid sulfur and carbon dioxide directly from the seafloor, and microbial communities draw energy from those chemical reactions through a process called chemosynthesis. Heterotrophic and chemoautotrophic bacteria are the dominant form of life throughout the hadal zone. Some researchers believe these vent environments may resemble the conditions where life on Earth first started billions of years ago.
No giant squid, colossal squid, or sperm whale has ever been confirmed at the true bottom of the Mariana Trench. What has been confirmed is a pale snailfish holding the depth record for any living fish, a supergiant amphipod, single-celled organisms the size of mangoes, and banned industrial chemicals that reached the deepest point on Earth decades before most people thought to look for them there.
