There is one big world ocean. It is split into five named parts: the Pacific, Atlantic, Indian, Arctic and Southern oceans. The Pacific is the biggest ocean. The Arctic Ocean is the smallest of the five oceans. As of November 2026, the Southern Ocean is the newest named ocean. It is the water around Antarctica.
Salt in the ocean mostly comes from rocks on land. Rain and rivers carry it to the sea. About 3.5 percent of the weight of sea water is dissolved salt.
Sunlight reaches only about the top 200 meters (656 feet) of the ocean. Deep down in the ocean it is dark.
As of November 2026, the deepest part of the ocean, the Challenger Deep, is about 11 kilometers (almost 7 miles) down. Pressure in the deepest trenches is more than 1,000 times the pressure at the surface, and the water is just above freezing. As of November 2026, most of the deep ocean has never been seen by people.
The ocean floor has tall mountains and deep valleys called trenches. The mid-ocean ridge is a chain of undersea mountains about 40,000 miles (65,000 kilometers) long that winds around the whole Earth.
The ocean water rises and falls along the shore. This is called the tide. The moon pulls the ocean into two bulges, one on the side facing the moon and one on the far side. As Earth turns, coasts pass through the bulges, which gives high tides. Twice a month the sun, moon and Earth line up, and the tides are a little higher and lower than usual. These are called spring tides.
A tsunami is a set of giant waves, most often made by an earthquake under the sea. A tsunami moves as fast as a jet plane over deep water. It slows down and grows taller as it reaches shallow water near land.
Corals are animals. They catch food with tiny tentacles and share their bodies with tiny algae. A coral that has lost its algae turns white. This is called bleaching, and a bleached coral is not dead, but it is under stress.
Much of the trash in the ocean starts on land. Microplastics are bits of plastic less than five millimeters long, about the size of a pencil eraser. Sea animals and birds can mistake plastic for food.
Phytoplankton are tiny plant-like living things in the ocean. They use sunlight to make food and give off oxygen. Many sea animals depend on them for food. About half of the oxygen made on Earth comes from the ocean, mostly from tiny plankton.
Words to know
- bleaching: turning white
- tsunami: giant waves
- Phytoplankton: tiny plant-like life
