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Place 04 · The Ridge and the Trench · Chapter 5 of 7
The Weight of the Water
At the bottom of the Challenger Deep the ocean presses on everything with more than 1,000 times the pressure at the surface, and small shrimp-like amphipods live there anyway.
About 15 minutes · 1 video · 1 3D model · 2 quick activities
Start here
One more atmosphere every 10 meters
At sea level the whole sky presses on you with 1 atmosphere of pressure. Seawater is so much heavier that every 10 m of depth adds another atmosphere: at 1,000 m you feel 101 of them, and at the bottom of the Challenger Deep, almost 11 km down, more than 1,000. Below about 1,000 m no sunlight reaches at all, and that dark, aphotic water, most of it just a few degrees above freezing, fills more than half of the ocean.
Watch this
See it move
TED-Ed — The effects of underwater pressure on the body. Four minutes on what the weight of the water does to the air inside you, which is exactly why you should not try Chapter 5 without a submarine.
While you watch, look for
- What happens to the air spaces in your body as you go deeper
- Why coming back up too fast is dangerous
- Something you did not expect. Hold onto it.
The short version
Three things going on here
The squeeze
Air loses
Pressure is mostly a problem for air, because gas squashes and water hardly does. Double the pressure and a gas shrinks to half its volume, so a balloon of air taken down to 10 m shrinks to half its size, and at 1,000 m to about a hundredth. Scientists send Styrofoam cups down on deep dives for fun: the pressure squeezes the air out of the foam and they come back as tiny, doll-sized cups.
The fish
No air, no problem
Deep-sea animals mostly have no air spaces to crush: their bodies are largely water, which barely compresses. Their proteins still need help, and many deep fish pile up a molecule called TMAO, a piezolyte that keeps proteins working under pressure; deeper fish carry more of it. The deepest fish ever filmed, a snailfish, was swimming at 8,336 m in 2022, and no fish has been found in the deepest quarter of the ocean.
The visitors
Very few trips to the bottom
In 1960 Jacques Piccard and Don Walsh rode the bathyscaphe Trieste to the bottom of the Challenger Deep, stayed about 20 minutes and saw little through the cloud of mud they stirred up. Nobody went back until James Cameron dived alone in 2012. In 2019 Victor Vescovo made two dives in his submersible Limiting Factor, and since then dozens of people have been down.
Photos: Francesco Ungaro, Benjamin Farren, Diego F. Parra / Pexels
Play with it
Meet a fish that lives near the limit
This 3D model is the type specimen of the Mariana snailfish, Pseudoliparis swirei, the specimen scientists used to describe the species, found living about 6,000 to 8,000 m down in the Mariana Trench. Spin it around: it looks soft, pale and a little see-through, which is exactly how you survive where the water weighs about 800 atmospheres.
3D model: “Mariana Snailfish” by fishguy on Sketchfab (CC BY), a scan of the type specimen of Pseudoliparis swirei.
Two questions to answer while you are in there
- This fish has thin muscles, pale see-through skin and bones that never fully harden. Why might a soft, watery body work better at 8,000 m than a hard, heavy one?
- The Mariana snailfish is the top predator in parts of the trench and eats tiny crustaceans. If there is no sunlight down there, where does their food ultimately come from?
Try it yourself
Check yourself
Try it
Gets Crushed or Handles It Fine?
Send eight things down to the abyssal plain and decide what the pressure does to each.
Try it · interactive
Pressure Dive
Dive a submersible from the surface to the bottom of the Challenger Deep: the gauge climbs one atmosphere every 10 m, a Styrofoam cup and a balloon of air shrink, the light fades to black, and the animals that live at each depth drift into view.
Open the Pressure Dive in its own window (opens in a new tab).
Words you will hear
Four terms, that is all
Hydrostatic pressure
Pressure from the weight of water above
Atmosphere (atm)
Pressure unit: the air's push at sea level
Aphotic zone
Deep, dark water that sunlight never reaches
Piezolyte
Molecule that protects proteins from deep-sea pressure
Check your understanding
Practice the whole chapter
Everything in this chapter, in eight quick questions. Retry as often as you like. Read the feedback on every answer, right or wrong, because that is where the learning is.
Practice
Practice: The Weight of the Water
Eight questions covering the video, the three cards, the four terms and the interactive above.
If you want more
Go deeper
- Webb 6.1 — Pressure — LibreTexts · 5 minutes
- Our World Ocean 3.4 — The Challenge of Pressure — LibreTexts · 5 minutes
- Our World Ocean 16.4 — Pelagic Depth Zones — LibreTexts · 20 minutes
Photo credits (Pexels): Blaque X, Skyler Sion, Francesco Ungaro, Benjamin Farren, Diego F. Parra.