It covers most of our planet. But how much do you really know about it? 

The ocean is crashing up on rocks, splashing onto shores, and lapping up against beaches all around the world. Its waters can be shallow enough to wade in or miles deep. And its temperature can be bathwater warm or freezing cold.

What we think of as the ocean is actually five different oceans: the Pacific, Atlantic, Indian, Arctic, and Southern. But they make up one big body of water. If you put your finger on a globe, you could make it “swim” through all five oceans without ever lifting it.

Today, the ocean covers nearly three-quarters of the Earth. But when the Earth formed, about 4.5 billion years ago, it had no ocean at all. No one knows for sure how the ocean came to be. But scientists have different ideas. One idea is that water in the form of a gas, called vapor, turned to liquid and fell as rain, eventually filling the ocean. Another idea is that water-filled comets and asteroids kept smashing into the Earth. After millions of years, the water from them filled the ocean.

There’s something else the ocean is full of: salt. If all the salt were taken out of the ocean and spread across the land, it would form a layer as high as a 40-story building. That’s a whole lot of salt! But how did it get there? Most of the salt in the ocean comes form the land. Rivers and streams wash away tiny amounts of salt from rocks and soil. This flows into the ocean. Also, water is always evaporating (turning to gas and rising into the air) from the ocean surface, leaving the salt behind. Millions of years of these things happening have resulted in today’s super-salty ocean.

But the ocean is more than just water and salt. What else is it full of? Keep reading!

ZONES OF LIFE

The ocean is full of life, from huge whales to tiny creatures too small to see without a microscope. Different kinds of creatures are found in different zones of the ocean:

THE SUNLIT ZONE (surface to 330 feet)
This is the brightest and warmest of the zones. Plants and algae (AL-jee) can grow here, and it is home to a huge variety of animals. It’s the shallowest of the zones, but is more crowded with creatures than the other zones.

THE TWILIGHT ZONE (330 feet to 3,300 feet)
Things get dimmer and colder as the water gets deeper. Plants and algae can’t survive here, and the animals in this zone are specially adapted to living in darkness. Many have big eyes to help them see. And some are bioluminescent (BY-oh-loo-min-ES-ent), which means they can make light with their bodies.They may use the light to attract prey or send signals to mates.

THE MIDNIGHT ZONE (3,300 feet to 36,000 feet)
Brrrr, you wouldn’t want to visit the midnight zone. The temperatures are close to freezing, and the water is pitch-black! Food is scarce and fewer animals live here. Ninety percent of the water in the ocean is in this zone.

THE GROUND FLOOR

FLAT AS A PANCAKE?
Which is flatter, this ray or the ocean floor? You might think both are fairly flat. But actually, a ray is much flatter than the bottom of the ocean. Though the ocean floor near the shoreline is mostly flat, it eventually drops off steeply. And if the water were all drained away, you’d see that the bottom has the same features that are on land—mountains, valleys, cliffs, and deep canyons.

DEEPER THAN DEEP
For a long time, scientists thought no life could survive in the deepest parts of the ocean. But then they discovered that vents, or cracks, in the ocean floor were home to hundreds of species, including special clams, shrimp, crabs, and red tube worms and eelpout fish shown here. Water heated deep in the Earth rises through the vents and gives the animals a warm place to live.

A PILE-UP OF POLYPS
A coral reef is like a big neighborhood of many kinds of creatures. Reefs can be miles long, but they are built by animals only about the size of peas! Each of these animals, called coral polyps (PAH-lips) builds a hard, stony cup to live in on the ocean floor. When the polyps die, the cups remain. Other polyps build more cups on top of these. After thousands of years of this, a reef forms. The biggest coral reef is the size of 70 million football fields!

AT THE BOTTOM OF IT ALL
The shallowest part of the ocean floor is called the continental shelf, which is like a ledge underneath the edge of the ocean. The deepest places are canyons called trenches. One of these, the Mariana Trench, is so deep that, if Mt. Everest sat at the bottom, there would still be more than a mile of water above its peak!

TALK ABOUT PRESSURE!
The deeper you go into the ocean, the more water there is above, pressing down on you. On parts of the ocean floor, this water pressure would feel like the weight of an elephant standing on your big toe—except that you’d feel that much pressure everywhere against your body!

EXTREME OCEAN ANIMALS

BIGGEST
The blue whale is the most gigantic animal in the ocean—and in the world! Everything about this whale is big: its body (as long as three school buses), its appetite (it eats 40 million shrimp-like krill a day), and even its tongue (as heavy as a small car)!

VIDEO: WATCH A BLUE WHALE DIVE AND SWIM!

STICKIEST
A remora is a fish that hitches a ride on bigger fish, such as sharks. It has a special suction disk on top of its head that lets it stick to a shark to get a free ride. When the shark eats a meal, the remora and its buddies get to nibble the scraps, as the ones here will do.

PRICKLIEST
Those scary-looking spikes on the crown-of-thorns sea star are more than just sharp—they’re filled with poisonous venom. But the venom isn’t the scariest thing about these sea stars. They can cause big problems on a coral reef. Crown-of-thorns sea stars feed on coral. Normally that would be OK—everyone needs to eat, right? But sometimes these sea stars increase in numbers.When that happens, there are so many of them that they can kill enough coral to destroy big parts of a coral reef.

TRICKIEST
Instead of looking like a fish, the frogfish looks more like a colorful sponge. But that’s not the only way it’s tricky. Sticking out of the frogfish’s face is a “fishing rod” with a wiggly lure at the end. When a curious fish comes near the lure—GULP!—the hungry frogfish snaps it up in less than a second.

PEOPLE AND THE OCEAN

LOTS TO LEARN
People have always been fascinated by the ocean. They swim, snorkel, and dive in it to explore its beauty and learn about its amazing creatures.

But the ocean is also a place of mystery. Its waters have been explored only to a depth equal to the height of the Washington Monument. And only about one percent of the ocean floor has been seen. All together, that’s only about five percent of the entire ocean—just a drop in the ocean bucket! The other 95 percent has never been seen by humans. And while we know about 233,080 ocean species, scientists believe there may be as many as a million in all.

OCEAN DRUG STORE
Imagine a doctor telling a sick person, “Take two slugs and call me in the morning.” That wouldn’t really happen, but scientists are making medicines by copying chemicals found in sea slugs, sponges, and other ocean creatures. The medicines are used to treat cancer and other diseases. And chemicals found in the blood of horseshoe crabs like these can be used to detect bacteria that make people sick. So someday a sea animal may help save your life!

OCEAN 911
As you’re reading this, hundreds of millions of tons of plastic trash are floating through the ocean. Fishing nets, plastic bags, and other plastic trash take a very long time to break down. Sea animals can get tangled in it or die from eating it. But that’s just one problem people have caused for the ocean. Oil spills sicken or kill many kinds of wildlife, such as this oil-covered penguin. Overfishing
has led to some species of fish becoming endangered. And climate change is harming coral and other ocean creatures.

INTO THE DEPTHS
The ocean is not an easy place to explore. Most of it is dark and cold and deep. An it’s just SO BIG! But the invention of mini research submarines made it possible for scientists to dive to and study the deeper parts of the ocean. Today, people can even pay to take undersea tours in subs like the one shown here to experience the ocean up close and personal!

Now that you’ve had your own tour of the ocean, you can see what a very special place it is!