Mariana Trench

• 1. Introduction

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  The Mariana Trench is a place where the water above presses down with 1086 bar of pressure. That is about eight tons on every square inch. The temperature at the bottom sits between 1 and 4 degrees Celsius. It is the deepest oceanic trench on Earth, and for generations the only way to learn anything about its floor was to lower something into it and wait. How do you measure a spot that deep, when every reading depends on the water above it? What forces opened such a gash in the Pacific? Who has actually reached the bottom, and what did they find waiting there? And what have people already sent down, whether they meant to or not?

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• 2. A Crescent in the Pacific

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  About 200 kilometres east of the Mariana Islands, the seafloor curves into a crescent about 2550 kilometres long and 69 kilometres wide. Its lowest known point lies at the southern end of a small slot-shaped valley called the Challenger Deep. The maximum depth there is 10935 metres, give or take 6. Set Mount Everest against that figure and the contrast is stark. The deepest point sits more than 2 kilometres farther below sea level than the mountain's peak rises above it.

  The trench takes its name from the neighbouring islands, which the Spanish called Las Marianas to honour Queen Mariana of Austria. Those islands are part of an arc that sits on the Mariana plate, on the western side of the trench.

  In 2009, the United States created the Mariana Trench Marine National Monument from the parts of the trench inside its exclusive economic zone. The Challenger Deep is not part of it. That valley lies inside the exclusive economic zone of the Federated States of Micronesia. The gash itself, and the islands beside it, are the work of two plates meeting.

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• 3. Where Plates Collide

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  Some of the oldest oceanic crust on Earth, up to 170 million years old, forms the western edge of the Pacific plate. Age has made it cool and dense. Along the Izu-Bonin-Mariana subduction system, that heavy edge is thrust beneath the smaller, younger Mariana plate to the west. The Mariana plate rides higher, and the difference in height between the two is what opens the trench. The deepest stretch of the boundary between them is the Mariana Trench proper.

  The same movement also built the Mariana Islands, though only indirectly. Minerals in the sinking slab of Pacific plate carry trapped water down with them. As that water escapes, it causes flux melting in the upper mantle above, and the melt feeds volcanic islands.

  In 2012, researchers from Washington University in St. Louis and the Woods Hole Oceanographic Institution chose the trench for a seismic survey of the subsurface water cycle. Using ocean-bottom seismometers and hydrophones, they mapped structures as deep as 97 kilometres beneath the surface.

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• 4. Sounding the Depths

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  In 1875, the Challenger expedition lowered a weighted rope into the trench and recorded a depth of 4475 fathoms. Two years later, a map titled Tiefenkarte des Grossen Ozeans, published by Petermann, marked a Challenger Tief at the site of that sounding. In 1899, the converted collier USS Nero logged 5269 fathoms. In 1951, Challenger II, under Chief Scientist Thomas Gaskell, used echo sounding, a far more precise and much easier method than the old drag lines. It recorded 5960 fathoms at the place now called the Challenger Deep.

  In 1957, the Soviet vessel RV Vityaz reported 11034 metres at a location it dubbed the Mariana Hollow. In 1984, the Japanese survey vessel Takuyō used a narrow multi-beam echo sounder and reported 10924 metres, also given as 10920 metres, give or take 10. The remotely operated vehicle KAIKO then made a dive record of 10911 metres on the 24th of March 1995.

  Surveys between 1997 and 2001 turned up another spot of similar depth, possibly deeper. Scientists from the Hawaii Institute of Geophysics and Planetology found it with a sonar system towed behind a research ship near Guam. They named it the HMRG Deep, after the Hawaii Mapping Research Group.

  On the 1st of June 2009, the RV Kilo Moana, mothership of the Nereus vehicle, mapped a spot at 10971 metres. Its Simrad EM120 sonar was accurate to better than 0.2 percent of water depth, which implies an uncertainty of about 22 metres. In 2011, a US Navy hydrographic ship mapped the whole trench to 100 metre resolution. That survey revealed four rocky outcrops thought to be former seamounts.

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• 5. The First Dive

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  At 1:06 in the afternoon on the 23rd of January 1960, the bathyscaphe Trieste touched bottom in the Challenger Deep. Aboard were Don Walsh and Jacques Piccard. The craft was Swiss-designed, Italian-built and owned by the United States Navy. It sank using iron shot as ballast and relied on gasoline for buoyancy.

  Its instruments first indicated a depth of 37800 feet. That number was later revised to 35814 feet. The depth was never read off directly. It was estimated by converting the measured pressure, using calculations based on the density of the water from the surface down to the seabed.

  The next two vehicles to reach the bottom, Kaikō and Nereus, carried no one at all.

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• 6. Crews and Robots

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  The first three expeditions, Trieste and the robots Kaikō and Nereus, measured remarkably similar depths between 10902 and 10916 metres. The fourth came on the 26th of March 2012, when Canadian film director James Cameron dived in the submersible Deepsea Challenger. He reached 10908 metres.

  On the 28th of April 2019, Victor Vescovo set a new record of 10928 metres in the DSV Limiting Factor. It was a Triton 36000/2 built by Florida-based Triton Submarines. He dived four times between that date and the 5th of May, becoming the first person to enter the Challenger Deep more than once. In June 2020, he descended again to around 10935 metres.

  On the 8th of May 2020, the Russian autonomous vehicle Vityaz-D reached 10028 metres. It was built by Russian shipbuilders with scientific teams of the Russian Academy of Sciences, the Russian Foundation for Advanced Research Projects and the Pacific Fleet. It was the first autonomous underwater vehicle to operate at such extreme depths. The mission lasted more than 3 hours, not counting the descent and ascent.

  On the 10th of November 2020, the Chinese submersible Fendouzhe touched bottom at 10909 metres. By 2022, the trench had seen 22 crewed descents and seven uncrewed ones.

  In July 2022, Vescovo piloted Limiting Factor with Dawn Wright aboard, making her the first Black woman to explore the trench. As mission specialist, she used side-scan sonar to map the Western Pool. Caladan Oceanic, a private undersea technology company, funded the dive.

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• 7. Life Under Pressure

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  "The bottom appeared light and clear, a waste of firm diatomaceous ooze," Piccard said of what he saw from the Trieste. He and Walsh also reported a flatfish about 30 centimetres long and some shrimp, which astonished scientists given the pressure. Many marine biologists now doubt the flatfish. They suggest it may have been a sea cucumber. Later, the uncrewed Kaikō brought up mud samples, and tiny organisms were found living in them.

  In July 2011, an expedition released untethered landers called drop cams, fitted with digital video cameras and lights. They filmed jellyfish and many other creatures. They also found xenophyophores, gigantic single-celled foraminiferans more than 4 inches across. These are the largest known cells, and they can withstand high levels of heavy metals. They belong to the class monothalamea, which stands out for its size, its abundance on the seafloor and its role as host to other organisms. Researchers from the Scripps Institution of Oceanography later found monothalamea at a record depth of 10.6 kilometres.

  In December 2014, a new species of snailfish was filmed at 8145 metres, a record for the deepest fish seen on video. The same 2014 expedition filmed huge amphipods called supergiants, an example of deep-sea gigantism, where species grow larger than their shallow-water relatives. In May 2017, the newly named Mariana snailfish was filmed again, at 8178 metres.

  When Fendouzhe returned in 2024, it dived 23 times. Thousands of tubeworms and mollusks were reported at depths from 8,200 to 31,000 feet. Then, in 2025, three papers in Cell described life across the hadal zone, between 6,000 and 11,000 metres. One of those studies identified more than 7000 new microbial species in the trench, and 89 percent of them were new to science.

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• 8. What Reaches the Bottom

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  In 2016, a research expedition analysed scavenging crustaceans taken from depths between 7,841 and 10,250 metres. The animals carried extremely elevated levels of PCBs, a toxin banned in the 1970s, and the chemical was concentrated in the trench sediment at every depth sampled. Further work found that amphipods swallow microplastics, with every amphipod examined holding at least one piece of synthetic material in its stomach.

  In 2019, Vescovo reported finding a plastic bag and candy wrappers on the floor. That year, Scientific American also reported high levels of carbon-14 from nuclear bomb testing in the bodies of animals living in the trench. A beer bottle has since been seen in the Western Pool.

  Like other trenches, the Mariana has been proposed as a nuclear waste site, in the hope that subduction would eventually carry the waste into the mantle. In 1979, Japan planned to dump low-level nuclear waste near Maug, in the Northern Marianas. International law prohibits ocean dumping of nuclear waste. Subduction zones also produce very large megathrust earthquakes, whose effects on long-term disposal cannot be predicted.

  In July 2015, researchers from the National Oceanic and Atmospheric Administration, Oregon State University and the Coast Guard lowered a titanium-shelled hydrophone into the Challenger Deep. No hydrophone had ever been deployed deeper than a mile before. It could not be retrieved until November, and its storage was full within 23 days. Months of analysis surprised the experts. Among the recordings were earthquakes, typhoons and baleen whales, along with the engines of boats passing overhead.

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