• How deep is the ocean? At nearly seven miles, the deepest part is a mile longer than Mount Everest is tall.
  • Both manned and unmanned vessels have reached these depths, called Challenger Deep.
  • It was long thought nothing could live in the Mariana Trench, but robotic probes have revealed worms, shrimp, and microorganisms.

Almost three-quarters of our world is covered in saltwater, and, on average, the ocean is about 12,100 feet or 2.3 miles deep. But in certain places, the sea floor plummets to truly astonishing depths.

In the Atlantic Ocean, the Puerto Rico Trench, which lies directly north of its namesake, plunges more than 27,000 feet below the surface. The Indian Ocean’s deepest point is the Java Trench, a 2,000-mile chasm off the coast of Sumatra with depths of around 24,000 feet.

But it’s the Pacific Ocean that boasts the deepest waters on Planet Earth.

How Deep Is the Ocean?

About 125 miles east of the Mariana Islands—a U.S. territory north of Guam—lies the deepest place known to man. The Mariana Trench, a crescent-shaped depression on the floor of the Western Pacific, stretches about 1,500 miles long and 43 miles wide.

The Mariana Trench is a subduction zone, the spot where one tectonic plate slides under the other. The Pacific Plate, which makes up half of the trench (the Philippine Plate comprises the other), is made up of some of the oldest seafloor in the world, around 180 million years old, so it has been settling lower and lower for quite some time. Two other factors contribute to the Mariana Trench’s enormous depth. First, its remote location means it’s far from any rivers that might fill it up with sediment. Second, fault lines cut the Pacific Plate into narrow grooves near the Trench, allowing it to fold at a steeper angle than in other subduction zones.

At the southern end of the Mariana Trench, there is a small, narrow valley known as the Challenger Deep. It is named for the 1951 expedition that first recorded its depth—an astounding 36,201 feet, or about 6.8 miles. If Mount Everest were placed into the trench at this point, its peak would still be underwater by more than 1.2 miles.

What’s It Like Down There?

At nearly seven miles underwater, the pressure is around 1,000 times greater than what we experience at sea level. Water temperatures hover around freezing, and everything is shrouded in absolute darkness.

It may be cold and quiet, but the deepest part of the ocean, we’re learning, is a noisy place. In 2015, a team made up of researchers from the National Oceanic and Atmospheric Administration, the U.S. Coast Guard, and Oregon State University, dropped a hydrophone (a waterproof microphone) into the Challenger Deep. Within 23 days, the data capacity of the device was full. After analyzing the recordings, the researchers reported hearing natural phenomena like earthquakes, typhoons, and whale calls, as well as man-made noises like boat engines.

Have Humans Explored the Deepest Part of the Ocean?

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In 1875, the HMS Challenger measured the depths of the Challenger Deep using a weighted rope. The discovery, made by a crew circumnavigating the globe on a marine research expedition, was a completely serendipitous one after unpredictable wind blew the ship off its planned course. More than 75 years later, Challenger II surveyed the spot using echo sounding, an easier and much more accurate way to map the ocean floor. This survey confirmed the Challenger Deep as the deepest spot in the world, at more than 36,000 feet below the surface.

Only four divers have ever explored the deepest depths of Challenger Deep. In 1960, Jacques Piccard and U.S. Navy Lieutenant Don Walsh explored the Challenger Deep in a submersible called Trieste. The dive lasted only 20 minutes due to the extreme pressure, and it kicked up so much debris from the seafloor that the men were unable to take photos.

It took more than 50 years for the next adventurer to reach the Challenger Deep. Filmmaker James Cameron visited in 2012 in a submarine he designed himself. During the three-hour dive, immense pressure damaged Cameron’s equipment. Batteries and sonar equipment went dead, and the vessel’s thrusters malfunctioned.

Adventurer Victor Vescovo was the next to set a new depth record in 2019, reaching 10,928 meters in his Triton-built submarine DSV Limiting Factor on a solo dive. Since then, Vescovo has piloted numerous others to Challenger Deep.

Dozens of other unmanned research vessels have explored the Mariana Trench and the Challenger Deep, contributing to our growing, but still incomplete knowledge, about the deepest corner of our world.

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What Lives Down There?

one of the highlights of the dive, a dumbo octopus uses his ear like fins to slowly swim away – this coiled leg body posture has never been observed before in this species
One of at least 15 species of dumbo octopus, found at depths of at least 13,000 feet, uses its ear-like fins to swim. Dwelling in the Midnight Zone, they are the deepest-living octopuses ever found.
NOAA Ocean Explorer

Pressure at the bottom of the Challenger Deep is so great that calcium cannot exist except in solution, meaning that bones would theoretically dissolve at such depths. Because of this, scientists are skeptical that any fish or other vertebrate could survive there. However, robotic probes that have sampled the water and seabed of the Challenger Deep have returned worms, shrimp, and microorganisms.

Paradoxically, there are only trace amounts of life on the floor of the Challenger Deep, yet scientists believe life on Earth may have gotten its start in these depths. Deep, hydrothermal vents that spew mineral-rich seawater—like the ones found in the Mariana Trench—may have provided the ideal conditions for the origin of life on our planet. The chemical reactions facilitated by these vents could be responsible for the increasingly complex organic compounds that eventually evolved into the lifeforms familiar to us today.

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Ashley Stimpson

Ashley Stimpson is a freelance journalist who writes most often about science, conservation, and the outdoors. Her work has appeared in the Guardian, WIRED, Nat Geo, Atlas Obscura, and elsewhere. She lives in Columbia, Maryland, with her partner, their greyhound, and a very bad cat.