How Deep Is the Ocean? A Guide for Kids
Sunlight gives up about 200 metres down. The ocean keeps going for another 10,700. That single fact is the whole shape of the sea: a thin, bright, crowded lid over a cold black volume that makes up the biggest living space on the planet, and that almost nothing on land has a feel for. Numbers do not help much on their own - eleven kilometres is just a number. What helps is dropping familiar things into the water and watching how quickly they vanish.
Everything below comes from the 3Eyes Ocean Depths lesson - a 3D interactive where a child scrolls from the surface to Challenger Deep past whales, towers and submarines placed at their true depths. The recordings below are taken straight from it; the lesson itself is part of 3Eyes.
Five zones, and where the light goes
The ocean is divided by depth into layers, and the boundaries are set by light and pressure rather than by anything you could see.
| Zone | Depth | Light | What lives there |
|---|---|---|---|
| Sunlight (epipelagic) | 0 - 200 m | Enough for plants | Nearly all the fish you have heard of, whales, reefs |
| Twilight (mesopelagic) | 200 - 1,000 m | Faint blue, no plants | Lanternfish, squid, things that glow |
| Midnight (bathypelagic) | 1,000 - 4,000 m | None at all | Anglerfish, gulper eels, sperm whales visiting |
| Abyssal (abyssopelagic) | 4,000 - 6,000 m | None | Sea cucumbers, tripod fish, the flat sea floor |
| Hadal | 6,000 - 11,000 m | None | Only in the trenches: snailfish, amphipods |
The sunlight zone is where the plants are, so it is where almost everything eats. Below 200 metres there is not enough light for photosynthesis, and below about 1,000 metres there is no light at all. Everything living down there is either eating what sinks from above, eating each other, or - around hot vents - living on chemicals from inside the Earth.
Things you know, dropped into the sea
This is the way to make the depth real. Stand each object on the sea floor and see whether its top reaches the surface.
- A blue whale is 30 metres long. Tip it on end and it would not reach a third of the way to the bottom of the sunlight zone.
- The Statue of Liberty is 93 m to the torch. Fully drowned before the light even starts to fade.
- The Eiffel Tower, 330 m, sits in the twilight zone with 670 m of dim water above it before the first zone boundary.
- A nuclear submarine operates around 400 m - and is built to. Below roughly 700 m the hull would crush.
- The Titanic lies at 3,800 m, in permanent darkness, so deep that it took 73 years to find.
- Mount Everest, 8,849 m, dropped into Challenger Deep would have more than two kilometres of water over its summit.
The last one is the fact that lands. The tallest thing on land does not reach the surface of the deepest water.
What the pressure does
Every ten metres of water adds the weight of the whole atmosphere again. At 10 m you are under 2 atmospheres, at 100 m under 11, at the bottom of Challenger Deep under about 1,100 - roughly the weight of a car pressing on every square centimetre.
Air is the problem, not water. Water barely compresses, so a fish with no air spaces is fine at any depth. A human with lungs, a submarine with a crew compartment, a polystyrene cup - anything holding air gets squeezed. Deep-sea researchers famously send foam cups down with their instruments and get back perfect shrunken copies a third of the size. Animals that live deep either have no air in them at all or, like the sperm whale, collapse their lungs on the way down and let their ribs fold.
Challenger Deep
The deepest known point in the ocean is in the Mariana Trench, east of the Philippines: about 10,935 metres, nearly 11 km. Fewer people have been there than have walked on the Moon. The first were Jacques Piccard and Don Walsh in 1960, in a steel ball under a float full of petrol, and they stayed twenty minutes. James Cameron went alone in 2012; since 2019 a purpose-built submersible has made the trip routine enough that dozens have now gone.
What they found at the bottom was not empty. Small pale fish, shrimp-like amphipods, sea cucumbers - and, in the sediment, plastic. The sea floor at 11 km is alive, and it is not out of reach of what we throw away.
Why is so little of it explored?
Because it is dark, cold, crushing, and enormous. Around a quarter of the sea floor has been mapped in detail; the rest is known only to the resolution of a satellite guessing at the shape of the water above it. We have better maps of Mars.
The good news for a child is that this is not settled science with all the answers in the book. New species come up from the deep on almost every expedition. It is one of the few places left where "nobody knows" is the honest answer to most questions.
How to teach it in five minutes
Start with the light. Two hundred metres, then dark. Everything else follows from that.
Then drop things in. The Eiffel Tower is the best - tall enough to be impressive, short enough to disappear into the twilight zone with room to spare.
Then Everest. The moment a child realises the tallest mountain fits inside the deepest trench with two kilometres left over, the depth has arrived.
Finish with the cup. If you can find a polystyrene cup and a picture of a crushed one, do. Pressure is abstract until something you own gets smaller.
Where 3Eyes Fits
The ocean lesson above is one of about sixty interactive science lessons built into 3Eyes, which is a parental controls product for family computers - it filters the machine, approves sites rather than blocking them one at a time, and limits YouTube to channels you choose.
The lessons exist because a restriction with nothing behind it is a hard argument to win. A laptop with sixty science lessons and a tutor on it makes the limits part of something rather than the whole of it.
About two minutes to set up. See what it includes or create an account.
Frequently asked questions
How deep is the ocean on average? About 3,700 metres - deeper than most people guess, because the shallow coastal water we see is a thin fringe. Most of the ocean is midnight-zone dark from top to bottom.
What is the deepest point in the ocean? Challenger Deep in the Mariana Trench, at roughly 10,935 metres. Mount Everest dropped in would be covered by more than two kilometres of water.
What are the five ocean zones in order? Sunlight (0-200 m), twilight (200-1,000 m), midnight (1,000-4,000 m), abyssal (4,000-6,000 m) and hadal (6,000 m to the trench floors). The first two are named for light; the last three are all completely dark.
Why can't humans go to the bottom of the ocean? Pressure. At 11 km down the water presses with about 1,100 times the force of the air at sea level, which would crush lungs instantly. People have gone, but only inside a thick metal sphere built to take it.
How much of the ocean has been explored? Around a quarter of the sea floor is mapped in detail, and far less has been seen directly. It is fair to tell a child we know the surface of Mars better than the bottom of our own sea.