The Deep Sea in Numbers
How deep the deepest trench goes, what the pressure does down there, how much of the ocean we have actually mapped, and why most of the animals glow.
The ocean covers about 71% of the planet and averages 3,700 metres deep. Almost all of it is dark, cold and under enormous pressure, and we know less about it than about the surface of Mars.
The numbers
| Thing | Figure |
|---|---|
| Deepest known point (Challenger Deep, Mariana Trench) | about 10,935 m |
| Everest, for comparison | 8,849 m |
| Pressure at the bottom of Challenger Deep | over 1,000 atmospheres |
| Depth sunlight effectively runs out | about 200 m |
| Depth no sunlight reaches at all | about 1,000 m |
| Average ocean depth | about 3,700 m |
| Seafloor mapped in modern detail | roughly a quarter |
| Deep-sea animals that produce their own light | around three quarters |
What the pressure means
At the bottom of the Mariana Trench the pressure is more than a thousand times what you feel at sea level — roughly the weight of a large car pressing on every square centimetre. Submersibles that go there have hulls of titanium or thick acrylic, and the acrylic viewports visibly compress on the way down.
Why nearly everything glows
Below a few hundred metres, light is not something you receive, it is something you make. An estimated three quarters of deep-sea animals are bioluminescent, using it to lure prey, confuse predators, signal to mates or produce a cloud of light and swim away from it. Most of it is blue, because blue travels furthest through seawater.
The zones
- Sunlight zone (0–200 m) — where almost all photosynthesis happens.
- Twilight zone (200–1,000 m) — enough light to see a silhouette, not enough to grow. Home to the biggest daily animal migration on Earth, as billions of creatures rise at dusk and sink at dawn.
- Midnight zone (1,000–4,000 m) — no light at all, near freezing.
- Abyss (4,000–6,000 m) — most of the deep seafloor.
- Trenches (6,000 m+) — the hadal zone, named after Hades, and only a tiny fraction of the ocean floor.
How little we have seen
Three people have been to the bottom of Challenger Deep by the old count, and a few dozen now that tourist submersibles reach it; twelve have walked on the Moon. Only around a quarter of the seafloor has been mapped at modern resolution, and new species turn up on almost every deep research cruise.