The deepest known point on Earth is a small section at the southern end of the Mariana Trench in the western Pacific Ocean called the Challenger Deep. The Challenger Deep is approximately 10,935 metres below sea level — about 11 kilometres straight down. Mount Everest, at 8,849 metres, would fit entirely within the Challenger Deep with more than 2 kilometres of water still above its summit. The pressure at the bottom is around 1,100 atmospheres, equivalent to roughly 1.1 tonnes per square centimetre. Five separate human missions and a handful of robotic ones have reached the bottom; for comparison, more than 600 people have stood on the Moon's nearby surface or in orbit around it, and over 7,000 have summited Everest. The Mariana Trench is, by most measures, the least-visited point on Earth that anyone has ever gone to.
The Geography
The Mariana Trench is a crescent-shaped depression in the seafloor of the western Pacific, running roughly 2,550 kilometres in a north-south arc east of the Mariana Islands (Guam, the Northern Marianas, and a string of smaller islets). The trench is around 70 kilometres wide. The Challenger Deep, the deepest single section, sits near the trench's southern end about 320 kilometres southwest of Guam. The trench is the surface expression of a subduction zone where the Pacific tectonic plate is descending beneath the smaller Mariana Plate, which sits east of the Philippine Plate. The descent of the Pacific Plate produces the trench at the surface and feeds the volcanic activity that built up the Mariana Islands themselves over the past 50 million years or so.
The Discovery
The Challenger Deep was discovered and named by the British survey ship HMS Challenger in 1875, during one of the foundational oceanographic expeditions of the late 19th century. Challenger used a hemp-rope sounding line — a literal weighted rope dropped over the side until it stopped paying out — to measure the depth. The crew recorded a depth of 8,184 metres, which substantially underestimated the actual depth (the rope must not have reached bottom or must have been measured imperfectly), but the figure was the deepest reading the expedition obtained anywhere and established that something unusually deep existed in this region of the Pacific. The actual modern depth measurements have been progressively refined since: the 1951 HMS Challenger II expedition gave 10,900 metres; the 1957 Soviet survey vessel Vityaz gave 11,034 metres; modern sonar surveys generally give 10,920 to 10,950 metres, with the precise figure depending on tide stage, salinity assumptions, and instrumental calibration. The depth is constantly being refined.
The Naming and the Maps
The Mariana Trench was for many decades known only from scattered deep soundings. The full mapping of the trench in modern detail did not happen until the multibeam sonar surveys of the 1960s through 1990s. Even today, the bathymetric maps of the trench have considerable uncertainty in many sections; the seafloor is not as well-mapped as the surface of Mars. The "Challenger Deep" was named for the original 1875 expedition; other named deeps along the trench include the "Sirena Deep" and the "HMRG Deep," each of them slightly shallower than the Challenger Deep but representing local maxima. The deepest point in any given trench can shift over time as tectonic processes and sediment fluxes change the bathymetry, but the Challenger Deep has been remarkably stable as the deepest known point for the past century and a half.
The First Descent
The first human descent into the Challenger Deep was made on 23 January 1960 by the bathyscaphe Trieste, with Swiss oceanographer Jacques Piccard and US Navy Lieutenant Don Walsh aboard. The Trieste was a deep-diving submersible designed by Piccard's father Auguste Piccard. Its main hull was a 2.16-metre-diameter steel sphere with walls 13 centimetres thick, surrounded by an external float chamber filled with petrol (which is incompressible and lighter than water, providing buoyancy at extreme depth where compressed-air ballast tanks could not function). The descent took 4 hours 47 minutes; the ascent took 3 hours 15 minutes. The two men spent approximately 20 minutes on the bottom. During the descent, an outer window cracked under pressure at around 9,000 metres; the inner pressure hull held. The depth they recorded was 10,916 metres. The next human descent did not happen for another 52 years.
The Recent Descents
The next person to reach the bottom of the Challenger Deep was the Canadian film director James Cameron in 2012, in his solo submersible Deepsea Challenger. Cameron's descent was the first solo crewed dive to the deepest point. The submersible was a specialised vertical-orientation craft about 7 metres tall with an extremely cramped pressure sphere. The descent took 2 hours 36 minutes; Cameron spent about 3 hours on the bottom collecting samples and footage; the ascent took 70 minutes. Several subsequent missions have followed. The American businessman Victor Vescovo descended to the bottom in 2019 in his submersible DSV Limiting Factor, and made several further dives to the area; he was joined on some by other passengers including astronaut Kathy Sullivan (the first woman to reach the bottom). Through 2025, fewer than 30 individuals have made the descent.
The Subduction Zone
The trench is the surface expression of a major subduction zone. The Pacific tectonic plate is moving westward at roughly 9 centimetres per year and is descending beneath the Mariana Plate at a steep angle — one of the steepest subduction angles anywhere on Earth. The descending Pacific Plate slab has been traced by seismic tomography down to depths of around 1,000 kilometres into the lower mantle. The friction and melting along the subduction interface produces the magma that feeds the volcanism of the Mariana arc, which built up the Mariana islands and continues to produce eruptions today. The 2009 eruption of the underwater volcano West Mata, in the same regional subduction system but slightly south, was one of the deepest active volcanic eruptions ever directly observed by submersible.
Life in the Deep
The Challenger Deep is genuinely inhabited. Cameron's 2012 dive observed amphipods (small shrimp-like crustaceans) on the bottom; subsequent dives have recovered specimens of amphipods that were genetically distinct from any previously identified species. Microbial life is abundant in the deep trench sediments — some of it living chemosynthetically off hydrothermal seeps along the trench wall, some of it living on organic material that has sunk from the surface. The deep-sea fish Pseudoliparis swirei, a snailfish, was filmed at about 7,966 metres in the Mariana Trench in 2014 and is thought to live close to the maximum depth at which fish anatomy can function (deeper than that, fish proteins denature under pressure even in evolved species). Various other unusual organisms — sea cucumbers, foraminifera, jellyfish — have been documented in the deep trench. The ecosystems are sparse compared to shallower depths but are not empty.
The Pollution Question
One of the unwelcome features of the trench is that even at the deepest accessible point on Earth, plastic pollution has been documented. A plastic bag was photographed at 10,898 metres during a 1998 deep-sea expedition. Subsequent dives have found increasing concentrations of microplastics, plastic film fragments, and other anthropogenic debris in the trench sediments. The 2019 dive by Victor Vescovo recovered what appeared to be a plastic bag from a depth of 10,927 metres. Plastic pollution apparently reaches the deepest places on Earth before it breaks down or is removed by ocean processes — a phenomenon that has dominated some of the public discussion of recent Mariana Trench expeditions.
The Mariana Islands and the People
The trench is named for the Mariana Islands, which it parallels along its length. The islands themselves are inhabited and include Guam (a US unincorporated territory, with around 170,000 residents and a substantial American military presence) and the Northern Mariana Islands (a US commonwealth, with around 50,000 residents). The indigenous population of the islands is the Chamorro people, who have inhabited them for at least 4,000 years. The islands have been under Spanish, German, Japanese, and now American sovereignty in succession, with each transition producing substantial demographic and political change. The Chamorro language survives but is endangered. The local economy depends substantially on the US military presence on Guam and on tourism from Japan, South Korea, and the Philippines.
The Long View
The Mariana Trench will continue deepening as the subduction process continues. The Pacific Plate will continue descending beneath the Mariana Plate at roughly 9 centimetres per year for the foreseeable geological future. The Mariana Islands will continue to be volcanically active. Some new volcanic islands will emerge over the coming millennia; some existing ones will erode and disappear. Eventually, on a geological timescale of tens of millions of years, the entire subduction zone may shift or stabilise, and the trench's depth and character will change accordingly. For human observers, however, the trench will remain what it has been since it was first sounded in 1875: the deepest known place on Earth, and the most difficult routine destination accessible to current technology, and the answer to one of the simpler-sounding but harder-to-actually-reach questions in geography.