The ocean is a huge body of salty water. The ocean covers most of Earth. There is one big world ocean. It has five named parts. They are the Pacific, Atlantic, Indian, Arctic and Southern oceans. The Pacific is the biggest ocean. The Arctic Ocean is the smallest of the five oceans.

Salt in the ocean mostly comes from rocks on land. Rain and rivers carry it to the sea.

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 is about 11 kilometers (almost 7 miles) down. This is the Challenger Deep. It is in the Pacific Ocean. As of November 2026, most of the deep ocean has never been seen by people.

The ocean floor has tall mountains. It has deep valleys called trenches. The mid-ocean ridge is a chain of undersea mountains. It is about 40,000 miles (65,000 kilometers) long. It winds around the whole Earth.

Coral reefs are built by tiny animals called polyps. Corals are animals. They catch food with tiny tentacles. They share their bodies with tiny algae.

The ocean water rises and falls along the shore. This is called the tide. The moon pulls the ocean into two bulges. One is on the side facing the moon. One is on the far side. As Earth turns, coasts pass through the bulges. This gives high tides.

The ocean soaks up heat from the sun and moves it around the planet. Water can soak up a lot of heat before it gets hot. So the ocean warms slowly. A hurricane needs sea water that is at least 80 degrees Fahrenheit (27 degrees Celsius) to form.

Much of the trash in the ocean starts on land. Microplastics are bits of plastic. They are less than five millimeters long. They are about the size of a pencil eraser. Sea animals and birds can mistake plastic for food.

About half of the oxygen made on Earth comes from the ocean, mostly from tiny plankton.

Words to know

• trenches: deep valleys
• tentacles: coral's food catchers
• bulges: raised parts

The short version

The ocean is split into five named parts, and the Pacific is the biggest. Salt in the ocean mostly comes from rocks on land. As of November 2026, the deepest part of the ocean, the Challenger Deep, is about 11 kilometers (almost 7 miles) down. Deep down in the ocean it is dark.

Quick facts

• The Pacific is the biggest ocean.
• The Arctic Ocean is the smallest of the five oceans.
• As of November 2026, the Challenger Deep is about 11 kilometers (almost 7 miles) down.
• Sunlight reaches only about the top 200 meters (656 feet) of the ocean.
• About half of the oxygen made on Earth comes from the ocean, mostly from tiny plankton.