How Big Are They?


The ocean covers almost 75 percent of the Earth—it’s crashing, splashing, and lapping up against every continent! The ocean’s waters can be shallow enough to dip just your toe into, or many miles deep. And its temperature can be as warm as bath water or icy cold. But the ocean is much more than just water. Ready to learn more? Then dive in!

Five in One

What we call the ocean is actually five different oceans: the Pacific, Atlantic, Indian, Arctic, and Southern. But though these oceans have different names, they actually make up one continuous body of water. So if you got into a boat in the ocean nearest you, you could travel through all five oceans without ever getting out of the boat. Smaller bodies of water are called seas, and they are usually partly surrounded by land.

An Ocean is Born

When the Earth was first formed about 4.5 billion years ago, it had no ocean at all. The early Earth was very hot, and the only water that existed was in the form of water vapor in the atmosphere. But as the Earth cooled, the water vapor began to condense, falling as rain. Over millions of years, enough rain fell to fill the ocean.

Rock Bottom

For years people thought the bottom of the ocean was a vast, flat plain. But if all the water was drained away, you’d see that the bottom has many of the same features we have on land. There are mountains, canyons, plains, plateaus, and very deep trenches. The most famous trench is the Mariana Trench, which is in the floor of the Pacific Ocean east of the Mariana Islands. The bottom of this trench is the deepest place on Earth! It is called the Challenger Deep, and it is almost 11.265 km deep. If Mt. Everest was at the bottom of it, there would still be more than 1.61 km of water above its peak!

More Than a Pinch

The ocean is famous for being salty. It has been estimated that if all the salt were taken out of the ocean, it would be enough to cover the Earth’s land surfaces with a layer as high as a 40-story building. Now that’s a lot of salt! But where does the salt come from? When water flows in rivers and streams, it washes away tiny amounts of salt from rocks and soil. Eventually this water flows into the ocean. At the same time, water is always evaporating from the ocean, while the salt remains. Millions of years of these two processes have resulted in the super-salty ocean of today.

Top to Bottom

Besides water and salt, what is the ocean full of? Life! The ocean is home to a huge variety of animals, from tiny plankton too small to see to the gigantic blue whale, as long as three school buses. The waters of the ocean are divided into five zones of life:

The Sunlit Zone (0 to 200 m)

This is the brightest and warmest of the zones. Though it is the smallest zone, it contains the most life. Plants are able to grow here and it is home to many animals, including coral, fish (including sharks and rays), jellyfish, sea stars, sea turtles, seals, dolphins, and many whales.

Twilight Zone (200 to 1,000 m)

At the top of this shadowy zone, there is enough light to see in daytime, but not enough light for plants to survive. The amount of light decreases greatly as the water gets deeper. The temperature is cold here, and the water pressure is high. The animals that live here are adapted to these conditions. Many have large eyes to help them see in the darkened waters, and a number of them are bioluminescent, which means they can make their own light. They use the light for different reasons, such as finding mates or attracting prey. Animals that live in the twilight zone include lancet fish, lantern fish, and some squid.

Midnight Zone (1,000 to 4,000 m)

Life is cold and difficult in the midnight zone. There is no light, and the water pressure is greatly increased. There are fewer animals living in this zone. Food is scarce; so many predators sit and wait for prey to come near, rather than using energy by chasing. Some try to trick prey into coming close by waving glowing lures to attract them. Many of the fish that live in this zone are bizarre-looking creatures, with huge fangs and gigantic mouths. The animals that live here include anglerfish, gulper eels, and viperfish.

Abyssal Zone (4,000 to 6,000 m)

This zone is pitch-black with near freezing temperatures. It is a very harsh environment and not many creatures live in it. But scientists have discovered one exception to this. Large communities of animals live around openings in the sea floor called hydrothermal vents. The water around these vents is much warmer than the surrounding water because it is heated by magma under the ocean floor. Tube worms, huge clams, and shrimp are some of the animals that live around the vents.

Trench Zone (6,000 to the bottom of the deepest parts of the ocean)

These areas are inside the trenches at the bottom of the ocean floor. The water pressure here is about as strong as the weight of 50 jumbo jets! But even still, some life exists. Scientists have found foraminifera, single-celled organisms, living at the bottom of the Mariana Trench, the deepest place in the world!

Visiting the Deep

The sea has fascinated people for centuries, yet more than 95 percent of the ocean remains unexplored. But by using vehicles called submersibles, scientists have learned much about the deepest parts of the ocean. Submersibles have super-strong hulls so they can go much deeper than submarines. The first submersibles were circular chambers lowered on cables from ships. Modern submersibles have motors and propellers and can be driven. One of the most famous submersibles is the Alvin, which was the first to carry passengers. While diving in Alvin, scientists discovered the first hydrothermal vents in the 1970s. Today the deepest-diving submersibles in the world are the Nautile, Mir I , Mir II, and the Shinkai, which all can dive deeper than 6,000 m!

Lessons from the Ocean

The study of the ocean is called oceanography. But oceanography includes many different areas of study, including chemistry, geology, physics, and biology. Many scientists work for groups that study the ocean, such as the National Oceanic and Atmospheric Administration (NOAA), the Commonwealth Scientific and Industrial Research Organisation (CSIRO), and the Intergovernmental Oceanographic Commission (IOC). By studying and monitoring the ocean, scientists can gather information about a multitude of things, from climate change to fish populations to weather forecasting to the effects of pollution.

Rough Seas

Unfortunately, the ocean faces some serious problems. Pollution from industrial waste, oil spills, and garbage dumping endangers and even kills marine life. Overfishing threatens fish populations and hunting has left many whales in danger of disappearing. But in the midst of all this bad news is some good news. All around the world, scientists, governments, conservation organizations, and other groups are working to help save the ocean. Laws have been passed that ban garbage dumping and other pollution. And regulations now protect many kinds of marine life. With everyone working together to take care of our beautiful ocean, the future may look brighter after all.