How deep is the Mariana Trench? The ocean's deepest point

The Mariana Trench is about 10,900 to 10,935 m deep at its deepest point, the Challenger Deep, in the western Pacific southwest of Guam. That is nearly 11 km, more than the height of Mount Everest above sea level. Surveys disagree on the exact figure by a few tens of meters; the best recent estimate, from submersible dives in 2020, is 10,935 m, give or take 6 m. The water pressure at the bottom is over 1,000 times that at the surface.

In short

• The Challenger Deep is the deepest known point in any ocean, about 10.9 km down; different instruments put it within a few tens of meters of each other.
• The trench marks where the old, heavy Pacific Plate dives under a younger plate to the west, part of the Ring of Fire.
• Only a few dozen people have been to the bottom, starting in 1960, and they found life there: amphipods, sea cucumbers and giant single-celled creatures, and also plastic.

Why do measurements of its depth differ?

Measuring the bottom of the sea from the top is harder than it sounds. Most measurements send a pulse of sound down and time the echo. But sound travels at different speeds in water of different temperature, saltiness and pressure, so turning a time into a distance needs a profile of the whole water column, and small errors over 11 km add up. The ship also has to be exactly over the deepest spot, which is a narrow pit in a trench floor of several basins. And "sea level" itself shifts with tides and waves.

The main results over time, from the history summarized on Wikipedia:

Year	Who	Depth reported	How
1951	British survey ship HMS Challenger II	5,940 fathoms, about 10,860 m	Echo sounding
1960	Bathyscaphe Trieste	First read as 11,521 m, later corrected to about 10,916 m	Depth gauge on the craft
2010	University of New Hampshire mapping team aboard USNS Sumner	10,994 m, plus or minus 40 m	Multibeam sonar
2012	James Cameron's Deepsea Challenger	10,908 m	Pressure sensor on the craft
2019	DSV Limiting Factor	10,925 m, plus or minus 4 m	Pressure sensor
2020 dives, published 2021	Samuel Greenaway, Kathryn Sullivan and colleagues	10,935 m, plus or minus 6 m	Sonar checked against direct pressure readings

No single figure to the meter is final. The 2021 estimate, published by NOAA and partners, is the most carefully corrected so far, and NOAA's National Ocean Service gives the depth as approximately 10,935 m. The spread in the table is about 0.5% of the total, which is small for a spot nobody can see.

How great is the pressure at the bottom?

Water is heavy. Every 10 m of seawater adds about 1 atmosphere of pressure, the pressure of the whole atmosphere at sea level. That gives a quick estimate:

1. The depth is about 10,935 m.
2. 10,935 ÷ 10 ≈ 1,094 atmospheres from the water.
3. Add the 1 atmosphere of air on top: about 1,095 atmospheres.
4. In more precise terms, pressure = density × gravity × depth. Seawater's density is about 1,025 kg per cubic meter at the surface and a little more at depth, because it is squeezed. Taking 1,050 kg/m³ as an average: 1,050 × 9.81 × 10,935 ≈ 113 million pascals, or about 1,100 atmospheres. The two answers agree.
5. One atmosphere is about 1 kg pressing on every square centimeter, so the bottom pushes with roughly 1,100 kg on each square centimeter, the weight of a small car on a patch the size of a fingernail.

That is why only hulls of thick steel or titanium, or flexible bodies full of water with no air spaces, survive there. Animals at that depth have no gas-filled organs to crush, and their cells are adapted to keep proteins working under pressure.

How big is it compared with Everest?

Put Mount Everest on the floor of the Challenger Deep. Its summit, at 8,849 m, would still be about 10,935 − 8,849 = 2,086 m below the surface: more than 2 km of water over the top of the world's highest mountain. The trench is far deeper than the ocean's average depth, about 3,682 m according to NOAA, or roughly three times as deep.

The trench is also long and narrow: about 2,550 km long but only about 69 km wide on average, a crescent-shaped scar east of the Mariana Islands. Its deepest part is a small area at the southern end.

How did the Mariana Trench form?

The trench is where one plate sinks beneath another. As the note on plate tectonics explains, ocean floor is made at mid-ocean ridges and destroyed at subduction zones. Here, the Pacific Plate, moving west, dives under the smaller Mariana Plate, part of the Philippine Sea Plate system. The Pacific floor at this edge is among the oldest ocean crust on Earth, up to about 170 million years old, and old ocean crust is cold and dense, so it sinks steeply and drags the seafloor down into an unusually deep trench.

The same sinking slab makes the Mariana Islands, a chain of volcanoes to the west fed by water released from the slab, and earthquakes that go hundreds of kilometers down. It is one of the most clear-cut stretches of the Pacific's Ring of Fire. The Atlantic has trenches too, but fewer and shallower: its deepest, the Puerto Rico Trench, reaches about 8,400 m near the edge of the area called the Bermuda Triangle.

Who has been to the bottom?

For 52 years after the first dive in 1960, only two people had been to the bottom. Submersibles built for repeated dives changed that: Wikipedia counted 27 people who had descended there as of July 2022, more than the 12 who have walked on the Moon.

Date	Craft	Who	What it showed
23 January 1960	Bathyscaphe Trieste, U.S. Navy	Jacques Piccard and Don Walsh	The bottom could be reached; they spent about 20 minutes there, in a cloud of stirred-up sediment
26 March 2012	Deepsea Challenger	James Cameron, alone	The first solo dive, with film and samples
28 April 2019	DSV Limiting Factor	Victor Vescovo	The first craft built to dive to full ocean depth again and again
7 June 2020	DSV Limiting Factor	Vescovo and Kathryn Sullivan, former astronaut and NOAA head	The first woman at the bottom, and the dives behind the 2021 depth estimate
10 November 2020	Fendouzhe (Striver), China	Three Chinese scientists	A live-streamed dive to a reported 10,909 m

What lives at the bottom?

Life is there, though sparse. The trench floor is cold, about 1 to 4 °C, completely dark, and fed only by what drifts down from above: dead plankton, fecal pellets and the odd fallen carcass. What lives there includes:

• Amphipods. Shrimp-like scavengers, some several centimeters long, that swarm bait within hours. A live one was first recovered from about 10,900 m in 1980.
• Sea cucumbers. Soft-bodied animals that crawl and swim over the sediment, eating it.
• Xenophyophores. Giant single-celled organisms, some as big as a fist, that build shells from sediment.
• Microbes. Bacteria and archaea in the mud, some living off chemicals rather than sunlight.

Fish don't reach the very bottom. Snailfish, pale and gelatinous, were filmed at about 8,100 m in the trench in 2014, and biologists think fish can't survive much below about 8,200 to 8,400 m because their cells can't counter the pressure beyond that.

Human traces got there first in some ways. A plastic bag has been recorded at about 10,900 m, and amphipods from the trench have been found carrying industrial pollutants. The deepest place on Earth is not out of reach of the surface. The seawater filling it carries salts washed off the land over billions of years, as the note on why the ocean is salty explains.

Check yourself

• Without looking back: why do careful measurements of the Challenger Deep still disagree by tens of meters?
• Work out the pressure at 5,000 m with the 10 m rule. How does it compare with the bottom of the Challenger Deep?
• Why is the Mariana Trench so much deeper than most trenches?

Sources

• How deep is the ocean? — NOAA National Ocean Service
• Revised depth of the Challenger Deep from submersible transects — Greenaway and others, Deep-Sea Research Part I, 2021, via NOAA
• Understanding plate motions, in This Dynamic Earth — U.S. Geological Survey
• Challenger Deep — Wikipedia
• Mariana Trench — Wikipedia