Mariana Trench

The Mariana Trench is a giant, deep valley in the floor of the ocean.

It is in the Pacific Ocean, near an island called Guam. It is the deepest place in the whole ocean. No sunlight reaches the bottom, so it is always dark and cold.

Far out in the western Pacific Ocean, near the islands of Guam and the Northern Mariana Islands, the sea floor drops into an enormous trench. The Mariana Trench curves for more than 2,540 kilometers, and on average it is about 69 kilometers wide.

Its deepest part is a narrow valley called the Challenger Deep, southwest of Guam. It is the deepest known place in the ocean. How deep exactly? That is surprisingly hard to measure, and different scientists get slightly different answers. NOAA, the US ocean agency, says about 10,935 meters. At that depth, the Woods Hole Oceanographic Institution (WHOI) explains, the water pushes more than 1,000 times harder than the air does at the surface.

Britannica describes the Mariana Trench as the deepest trench known on Earth, lying mostly east and south of the Mariana Islands in the western North Pacific: an arcing depression more than 2,540 km long with a mean width of 69 km. The greatest depths are in the Challenger Deep, a smaller, steep-walled valley on the trench floor southwest of Guam. The trench lies within the territories of the US dependencies of Guam and the Northern Mariana Islands and was designated a US national monument in 2009.

The depth of the Challenger Deep is a classic measurement problem. NOAA's National Ocean Service gives about 10,935 m; Greenaway and colleagues (2021), using pressure sensors on the submersible Limiting Factor during dives in 2020, reported 10,935 m ± 6 m at 95 percent confidence; WHOI gives 10,911 m; a 2023 multibeam survey by Japan's R/V Hakuho-maru found a maximum gridded depth of 10,927 m (Scientific Data, 2026); and Britannica records 10,994 m from a US team in 2011. Differences come from sound-speed models, pressure-sensor calibration and gravity corrections.

The ground under the ocean is made of giant pieces, like a puzzle.

Here, one piece slowly slides under another piece. Where it bends down, it makes a deep, deep trench. The sliding also makes volcanoes and islands nearby.

Earth's outer shell is broken into huge pieces called tectonic plates, which move very slowly. At the Mariana Trench, two plates push together. The old, heavy Pacific Plate slides down underneath the smaller Mariana Plate. This is called subduction.

As the Pacific Plate bends down into Earth's mantle, it pulls the sea floor into a steep, V-shaped trench. WHOI explains that subduction also makes melted rock rise up, building volcanoes and chains of volcanic islands next to the trench, like the Mariana Islands. Subduction zones like this one also cause many of the world's earthquakes, and they are part of the Pacific Ring of Fire.

WHOI explains that trenches form at subduction zones, where converging plates force the older, denser plate beneath the lighter one and down into the mantle, bending the lithosphere into a steep, V-shaped depression. Britannica places the Mariana Trench in the western Pacific system of trenches coinciding with subduction zones, where the Pacific Plate is forced below the Mariana Plate. The plate model used for our map (PB2002) marks the Pacific-Mariana boundary here as a subduction zone.

Subduction generates melt that rises to build volcanic arcs parallel to trenches, and subduction zones host much of Earth's seismicity, including some of the largest earthquakes on record. WHOI notes that trenches deeper than 6,000 m make up the hadal zone, which it says accounts for the deepest 45 percent of the global ocean, although the deepest parts of a trench cover only about 1 percent or less of its area. The Tonga, Kuril-Kamchatka, Philippine and Kermadec trenches also exceed 10,000 m.

Even in the deep, dark trench, there are living things!

A little see-through fish called the Mariana snailfish lives very deep down. It eats tiny shrimp-like animals. Food drifts down from far above.

The bottom of a trench has no sunlight, water just above freezing, and crushing pressure. Yet life is there. WHOI explains that food drifts down from the sunny surface and collects along the V-shaped bottom of trenches, so some specially adapted animals do very well.

In 2017, scientists named a new fish, the Mariana snailfish. It is small, see-through and has no scales, and it lives as deep as about 8,000 meters along the trench near Guam. Scientists collected it with traps that took about four hours to sink to the bottom. The University of Hawaiʻi says the pressure there is like an elephant standing on your thumb, yet the snailfish is the top hunter, eating tiny crustaceans.

WHOI describes hadal environments as having pressures more than 1,000 times those at the surface, near-freezing temperatures and no light for photosynthesis; extreme pressure, accumulation of food along trench axes and geographic isolation favor very high abundances of a few specialized organisms, some with pressure-resistant proteins, others drawing chemical energy from seeps and mud volcanoes. NOAA notes that pressure rises by one atmosphere for every 10.06 m of depth.

The Mariana snailfish (Pseudoliparis swirei), described in Zootaxa in 2017 by a team including University of Hawaiʻi and University of Washington researchers, was collected as 37 specimens from about 6,900 to 8,000 m during 2014 and 2017 R/V Falkor cruises; a Japanese team later filmed a snailfish at 8,178 m, which the university called the deepest sighting so far (2017). Co-author Thomas Linley noted that the trench's funnel shape concentrates food and that the snailfish, free of predators, is the top predator among abundant invertebrate prey.

Long ago, sailors on a ship called Challenger measured how deep the trench is.

In 1960, two explorers went down in a special craft and reached the bottom. Much later, more explorers went down too.

The first measurement of the trench was made in 1875 by the crew of the British ship HMS Challenger, which is how the Challenger Deep got its name. Later ships used better and better electronic equipment to measure the depth more carefully.

On January 23, 1960, Jacques Piccard and US Navy officer Don Walsh rode the bathyscaphe Trieste, a deep-diving craft, to the bottom of the Challenger Deep. More than 50 years passed before anyone went back. On March 26, 2012, filmmaker James Cameron dove there alone in the Deepsea Challenger, a submersible he helped design, and Don Walsh advised his team.

Britannica traces the measurement history: a 1875 Challenger Expedition sounding of 8,184 m near the trench's southern end; Nero Deep (9,660 m) in 1899; a 1957 sounding of 10,990 m by the Soviet ship Vityaz, later raised to 11,034 m; 10,924 m from a Japanese expedition in 1984; and 10,994 m from a US team in 2011. A separate deep, Sirena Deep south of Guam, first found in 1997, has been reported at 10,641 and 10,809 m.

On 23 January 1960 the French-built, US Navy-operated bathyscaphe Trieste carried Jacques Piccard and Don Walsh to 10,916 m (Britannica's figure) in the Challenger Deep. On 26 March 2012 James Cameron piloted Deepsea Challenger solo to 10,898 m. In 2020, dives by the submersible Limiting Factor carried precise pressure sensors that Greenaway and colleagues used to calculate the 10,935 m depth published in 2021.

People cannot swim down to the trench. It is far too deep.

Scientists send special robots and cameras instead. We can learn about the deep sea from their pictures.

Nobody can visit the bottom of the Mariana Trench without a very special craft built for the crushing pressure. NOAA explains that every 10 meters of water adds the weight of a whole extra atmosphere. So most exploring is done with robots, cameras and traps lowered on long cables or dropped with weights, like the snailfish traps that took four hours to reach the bottom.

The trench is protected too: the United States made it part of a national monument in 2009. You can explore it from home through photos and videos shared by ocean scientists, and learn how the deep sea connects to the ocean we swim in.

The Challenger Deep is accessible only to full-ocean-depth vehicles: crewed descents have been rare (among them Trieste in 1960, Deepsea Challenger in 2012 and the Limiting Factor dives of 2020), and WHOI notes that knowledge of trenches still rests heavily on mid-20th-century sampling campaigns and remote instruments. At about 1 atmosphere per 10 m (NOAA), the bottom pressure exceeds 1,000 atmospheres.

Research uses landers, baited traps released acoustically after 12 to 24 hours on the bottom, multibeam sonar from ships and autonomous or tethered robots. Britannica notes the trench's designation as a US national monument in 2009. For the public, exploration is virtual: survey data, published images and expedition video.