The ocean is the single, massive body of saltwater that covers most of our planet. When viewed from space, Earth looks like a blue marble because the ocean wraps around nearly 71 percent of the surface. It holds about 97 percent of all the water on Earth. While we often talk about separate areas like the Pacific or Atlantic, they are all connected, flowing into one another to form what scientists call the World Ocean.
The Biggest Bathtub on Earth
The ocean’s salty taste comes from a very long process involving the land all around us.
When rain falls, it absorbs carbon dioxide from the air, making it slightly acidic. This weak acid slowly breaks down rocks on land. As the rainwater flows into streams and rivers, it carries tiny bits of dissolved minerals, like sodium and chloride, from the broken-down rocks. Rivers eventually empty this water into the ocean.
All the rivers in the world constantly deliver these minerals to the ocean. Over millions of years, water evaporates from the ocean surface to form clouds, but the salt and minerals are left behind. This continuous cycle means the salt builds up, making the ocean water salty. Underwater volcanoes and vents on the seafloor also release extra minerals, adding to the ocean’s unique composition.
Climbing Down the Ocean Layers
Scientists divide the ocean vertically into zones based on how much sunlight reaches them. The environment changes dramatically with depth. The first layer, the Sunlight Zone (or Epipelagic Zone), extends about 660 feet down, which is as deep as a tall skyscraper is high.
This top zone is where most of the light and warmth are found, allowing tiny plant-like organisms called phytoplankton to grow. Below this is the Twilight Zone (Mesopelagic Zone), which reaches down to about 3,300 feet. Only faint, filtered light makes it here, making it too dim for plants to survive.
Finally, we reach the Midnight Zone (Bathypelagic Zone), where no sunlight penetrates and the water is dark. The temperature drops close to freezing, and the water pressure becomes crushing, similar to having a large car sitting on your hand. Animals living here have adaptations, like producing their own light, a process called bioluminescence, to help them navigate the darkness.
Meet the Neighbors
The ocean is home to an incredible variety of animals, from the microscopic to the enormous. In the Sunlit Zone, the abundance of light supports lush ecosystems like coral reefs, which function like vibrant, bustling underwater cities. Coral polyps, which are tiny animals, build these massive structures from calcium carbonate, providing homes for a quarter of all marine species.
Colorful fish like clownfish and parrotfish dart among the coral branches, while sea turtles and dolphins cruise the open, well-lit waters. Even the smallest creatures, like the drifting plankton, are the foundation of the entire ocean food web, feeding everything from tiny fish to massive whales.
As we move into the darker Twilight Zone, inhabitants often have large eyes to catch the dim light. Creatures like the hatchetfish and certain squids use bioluminescence to camouflage themselves or attract prey. Deeper still, in the Midnight Zone, animals like the anglerfish use a glowing lure on their head to hunt in the pitch black. Marine mammals, such as the sperm whale, are known to dive thousands of feet into these dark zones to hunt giant squid.
Why the Ocean is Our Best Friend
Tiny plant-like plankton in the surface waters absorb sunlight and produce oxygen, supplying at least half of the air we breathe. This means that every other breath you take comes thanks to the ocean. This process allows life on land to thrive.
It also plays a part in regulating our weather and climate. The vast amount of water absorbs heat from the sun, transporting it around the globe through large currents. This movement helps keep Earth’s temperatures stable and influences rainfall patterns necessary for life on land. The ocean is also a massive source of food for people around the world, making its health important for everyone.