There are five oceans on Earth: the Pacific, the Atlantic, the Indian, the Southern and the Arctic. Together they cover about 71% of the planet — nearly three quarters of the whole surface. If you could shrink Earth down to the size of a football, the water would be a film thinner than a sheet of paper, and almost everything alive would live inside it.
Meet the Five Oceans
The Pacific is the biggest by a long way. It covers about 165 million square kilometres, which is more than all the land on Earth put together. You could fit every continent inside it and still have room left over.
The Atlantic is second, and it is the one between the Americas and Europe and Africa. It is getting wider by about 2.5 centimetres a year — roughly the speed your fingernails grow — because the seafloor is spreading apart down the middle.
The Indian Ocean is the warmest. The Southern Ocean circles Antarctica and only officially got its name in 2000, which makes it younger than most of the people reading this. The Arctic is the smallest and the shallowest, and much of it is covered in floating ice.
It Is Really One Ocean
Here is something odd: the five oceans are all joined together. You could put a boat in the Arctic and sail to Antarctica without ever crossing land.
Scientists sometimes call it the World Ocean for that reason. The five names are useful for maps and for talking about different places, but the water itself moves between all of them.
Water actually travels a long way. A drop that starts near Greenland can take around a thousand years to make a full loop around the world and come back — a journey scientists call the global conveyor belt.
How Deep Does It Go?
The average depth of the ocean is about 3,700 metres. That is deeper than most mountains are tall.
The deepest place we know of is Challenger Deep, in the Mariana Trench in the western Pacific. It is roughly 10,935 metres down. If you dropped Mount Everest into it, the top of the mountain would still be about two kilometres under water.
Only a handful of people have ever been to the bottom. More people have walked on the Moon than have visited the deepest point of the ocean.
Why the Sea Is Salty
Rain falling on land picks up tiny amounts of salt and minerals from rocks and soil. Rivers carry them to the sea. Then the sun evaporates the water, but the salt stays behind.
This has been happening for billions of years, so the salt has built up. There is enough salt in the ocean to cover all the land on Earth in a layer about 150 metres thick.
Not all seas are equally salty. The Baltic Sea is barely salty at all because so many rivers pour into it, while the Red Sea is very salty because it is hot and few rivers reach it.
Try This at Home
Fill a glass with water and add three teaspoons of salt. Stir until it disappears. Now drop in an egg. It should float, because salty water is denser than fresh water.
Try the same egg in plain water and it will sink. That is exactly why it is easier to float in the sea than in a swimming pool, and why the Dead Sea — which is nine times saltier than the ocean — makes people bob on the surface like corks.
You can also try this: put a small amount of salty water in a saucer on a sunny windowsill. Come back in a few days. The water will be gone and the salt will be sitting there, which is how the ocean got salty in the first place.
The Ocean Makes the Air You Breathe
Most people think forests make our oxygen. Actually, the ocean makes somewhere between half and 80% of it.
The work is done by phytoplankton — tiny plant-like living things floating near the surface, far too small to see without a microscope. They use sunlight to make food, and they release oxygen while they do it, just like a tree does.
One type, called Prochlorococcus, may be the most common photosynthetic organism on Earth. There are more of them in the sea than there are stars in our galaxy, and nobody even knew they existed until 1986.
Waves, Tides and Currents
These three things sound similar and are completely different.
Waves are made by wind pushing on the surface. The water itself mostly stays in place and bobs in a circle — it is the energy that travels, not the water.
Tides are the sea rising and falling twice a day, and they are caused by the Moon's gravity pulling on the ocean. Currents are like rivers inside the sea, moving warm and cold water around the planet. The Gulf Stream carries warm water from the Caribbean towards Europe, which is why Britain is much milder than Canada even though they are at the same latitude.
Who Lives Down There
Scientists have named around 240,000 ocean species, and they think there could be a million or more still undiscovered.
The blue whale lives in the ocean and is the largest animal that has ever existed — bigger than any dinosaur. It can be 30 metres long and its heart is roughly the size of a small car.
Deep down where no light reaches, animals make their own. Anglerfish dangle a glowing lure in front of their mouths. Some squid squirt glowing clouds instead of ink. This is called bioluminescence, and most of the animals in the deep sea can do it.
The Twilight Zone and Below
Scientists split the ocean into layers by how much light gets in.
The sunlight zone is the top 200 metres, where almost all the plants and most of the fish you know about live. Below that is the twilight zone, down to 1,000 metres, where there is a little light but not enough for plants.
Then comes the midnight zone, which is completely dark and makes up most of the ocean by volume. Below that are the abyss and the trenches, where the pressure is so great it would crush a car flat. Animals live there anyway.
How Much of It Have We Seen?
We have better maps of the surface of Mars than of the bottom of our own ocean. Only about a quarter of the seafloor has been mapped in detail, and most of what we do know comes from satellites measuring tiny bumps in the height of the sea surface caused by mountains far below.
Sending anything to the deep sea is hard. At 10,000 metres down, the pressure is about a thousand times what it is at the surface — the same as balancing an elephant on your thumb. Submarines that go there have to be built like small tanks.
An international project called Seabed 2030 is trying to map the whole ocean floor by the end of this decade. Ships, submarines and even fishing boats are contributing data to it.
Five Oceans, or Four?
For most of the twentieth century, maps showed four oceans. The Southern Ocean was added by the International Hydrographic Organization in 2000, and National Geographic only started drawing it on maps in 2021.
Why add one? Because the water circling Antarctica really is different. A powerful current called the Antarctic Circumpolar Current runs all the way around the continent, and the cold water inside it does not mix easily with the warmer water outside. It behaves like its own ocean, so it got its own name.
Not everyone agrees on where it stops. Most definitions put the boundary at 60 degrees south, but some countries draw it further north, which means the Southern Ocean is a slightly different size depending on whose map you are looking at.
Ocean Records
The longest mountain range on Earth is underwater: the Mid-Ocean Ridge, running about 65,000 kilometres around the globe like the seam on a tennis ball.
The largest living structure is the Great Barrier Reef off Australia, made by billions of tiny coral animals over thousands of years. It is around 2,300 kilometres long and can be seen from space.
The biggest waterfall is also underwater, in the Denmark Strait between Greenland and Iceland, where cold dense water pours downwards for around 3,500 metres — more than three times the height of the tallest waterfall on land.
Looking After It
Around eight million tonnes of plastic enter the ocean every year. Some of it collects in huge slow-turning areas called gyres, and the best known is the Great Pacific Garbage Patch between Hawaii and California.
It is not an island of rubbish you could stand on. Most of it is tiny broken-up pieces spread through the water, which makes it much harder to clean up and much easier for animals to swallow.
The ocean is also getting warmer and slightly more acidic as it absorbs carbon dioxide from the air, which makes it harder for corals and shellfish to build their skeletons. Small things help: using less single-use plastic, picking up litter near water, and choosing seafood from sustainable sources. If you want to keep exploring, find out how many countries there are in the world or read about how islands form in the middle of all that water.