What if you could dive to the deepest trench?

Far out in the Pacific Ocean is a long, deep trench. Its deepest spot is called the Challenger Deep.

What if you could dive all the way to the bottom? What would you find?

The deepest place in the ocean is the Challenger Deep, at the southern end of the Mariana Trench in the western Pacific Ocean. NOAA says it is about 10,935 meters (35,876 feet) deep. If you put Mount Everest at the bottom, its peak would still be more than 2 kilometers underwater.

So what if you could dive all the way down? In this thought experiment you can follow three paths: the enormous pressure of the water, the surprising life in the deep, and the submarines that have already made it.

NOAA places the Challenger Deep at approximately 10,935 m (35,876 ft), in the southern Mariana Trench several hundred kilometers southwest of Guam; WHOI gives 10,911 m, a reminder that measurements of the deepest point differ slightly. It is named after HMS Challenger, whose crew first sounded the trench in 1875.

This thought experiment follows a descent into the hadal zone, the trench depths below 6,000 m, and asks three questions: how pressure changes, how life copes, and how engineers have built vehicles to reach the bottom.

Most of the ocean is deep. But trenches are the deepest places of all.

They form where one piece of Earth's crust slides under another.

The ocean is about 3,682 meters deep on average, NOAA says. But in a few places it plunges much deeper, into long, narrow valleys called trenches. Trenches form where one of Earth's tectonic plates slides beneath another.

The Mariana Trench is about 1,580 miles long but only about 43 miles wide on average. Several other trenches, such as the Tonga and Kermadec trenches, also reach deeper than 10,000 meters.

The mean ocean depth is about 3,682 m (NOAA). Trenches, steep depressions deeper than 6,000 m, form at subduction zones, where old ocean crust from one tectonic plate is pushed beneath another; at the Mariana Trench, the Pacific Plate subducts beneath the Philippine Plate (WHOI).

The Mariana Trench is about 1,580 miles long and averages about 43 miles wide. The Tonga, Kuril-Kamchatka, Philippine and Kermadec trenches also exceed 10,000 m. Trenches make up the hadal zone, the deepest 45 percent of the global ocean by depth range.

As you dive deeper, the water above you pushes harder and harder.

It also gets dark and cold. No sunlight reaches the bottom.

As you go down, more and more water is stacked above you, pushing down. NOAA says the pressure increases by one atmosphere, the push of all the air above us at sea level, for every 10.06 meters of depth. By the bottom of the Challenger Deep, that adds up to more than 1,000 times the pressure at the surface.

It is also completely dark and cold. Woods Hole Oceanographic Institution says the water in trenches stays just above freezing, and no light reaches down to let plants grow. Only a very strong, specially built submarine could carry you there safely.

Hydrostatic pressure rises by about one atmosphere (14.7 psi) per 10.06 m (NOAA). Over 10,935 m that is roughly 1,087 additional atmospheres, consistent with WHOI's statement that trench pressures exceed 1,000 times the surface value: about 16,000 pounds on every square inch.

WHOI also notes constant temperatures just above freezing and no light for photosynthesis. A crewed visit therefore requires a pressure hull: the Trieste carried its two-person crew in a small, pressure-resistant sphere, and the Deepsea Challenger in a narrow vertical vessel.

Some animals are made to live with the squeeze. Their bodies bend and do not break.

In the deep, special fish and small creatures find food that sinks from above.

Deep-sea animals are not crushed, because they do not have air spaces that the pressure can squeeze. NOAA explains that whales handle big pressure changes because their ribs are joined by bendy cartilage and their lungs can safely collapse.

In trenches, WHOI says, some animals have special proteins that resist the pressure, and many live on bits of food that drift down from the surface and collect at the bottom of the V-shaped trench. Snailfish, the deepest fish ever filmed, have no swim bladder, the gas-filled bag many fish use to float.

Pressure damages gas spaces most; animals without them, or with flexible structures, cope. NOAA describes whales' cartilage-bound ribs and collapsible lungs, letting sperm whales dive to 7,000 feet or more. WHOI reports hadal organisms with proteins and biomolecules suited to resisting hydrostatic pressure, others using chemical energy from seeps and mud volcanoes, and many relying on organic material funneled to trench axes.

Snailfish lack a swim bladder. Researchers led by Alan Jamieson hypothesized that fish cannot survive much below 8,200 to 8,400 m, where a pressure-protecting cell chemical reaches its limit.

In 1960, two explorers rode a deep-sea submarine called the Trieste to the bottom. In 2012, James Cameron dove there alone!

Today, robots help explore the deep too.

People have already been to the Challenger Deep. In 1960, U.S. Navy Lieutenant Don Walsh and Swiss oceanographer Jacques Piccard reached it in the Trieste. Fifty-two years later, in 2012, filmmaker James Cameron made the first solo dive there in the Deepsea Challenger.

When Cameron came back up, Walsh shook his hand and said, "Welcome to the club. There are just two of us." Walsh believed robots would do most of the work of exploring trenches, but that people would still want to go and see for themselves.

Walsh and Piccard reached the Challenger Deep in the Trieste in 1960, a gasoline-filled float above a small two-person pressure sphere (National Geographic Education). On 26 March 2012 James Cameron made the first solo dive in the vertically oriented Deepsea Challenger, with Walsh advising the team.

Exploration remains limited: WHOI notes much hadal knowledge still comes from 1950s sampling campaigns and remote samples, and it built the robotic vehicle Nereus for these depths. Walsh predicted uncrewed vehicles would do the heavy lifting, with room still for human observers.