The ocean is constantly moving. You can see this when waves crash onto shore. Swimmers can feel it when an ocean current pulls them along. Surface currents move water across the planet. They are powered by ocean winds. The ocean also has deep, underwater currents. These very strong currents slowly mix the waters of the whole ocean.

A process, or set of steps, drives underwater currents. This process is called thermohaline circulation.

Ocean Circulation

Thermohaline circulation moves a huge current of water around Earth. This current travels from north to south and back again. It slowly turns over the ocean's water from top to bottom. Surface waters move down. Deep waters are forced up.

The whole process is something like a giant conveyor belt. A conveyor belt is a continuously moving track. It circles back on itself. You may have seen conveyor belts at theme parks. A roller coaster is a type of conveyor belt.

Below are a few simple points about water. They will help you understand how ocean water moves:

• Water always flows down toward the lowest point.

• Cold water is heavier than warm water.

• Salt is heavy. So, the more salt in the water, the heavier it is.

• Ocean water always moves toward balance. For example, if surface water cools and becomes heavier, it will sink. The warmer water below will then rise. It balances out the missing surface water.

The Ocean Top to Bottom

The ocean has three layers. The top layer is about 100 meters (330 feet) deep. Enough sunlight reaches it to allow plant life.

The middle layer is called the thermocline. It is up to 1,000 meters (3,300 feet) deep.

Next is the bottom layer, or deep ocean. On average, it is about three kilometers (two miles) deep. It is is covered with the broken-down remains of plants and animals. This is known as sediment. It provides nutrients, or food, for countless life forms.

Currents Move Nutrients and Water Temperatures

Nutrients continuously sink from the ocean's surface to its depths. But their travel is not one-way. Over time, deep water rises to the surface. As it rises, it brings nutrients back up with it. These nutrients provide food for ocean life living near the surface.

Over time, the ocean slowly turns over. It flips from top to bottom. Like a conveyor belt, thermohaline circulation moves nutrients from one part of the ocean to another. How does this happen?

Let's start in the northern Atlantic Ocean. From there we will follow the conveyor belt as it moves water around Earth.

In the northern Atlantic Ocean, the water is cold. Because cold water is heavier, some of it sinks to the ocean floor. This water is known as the North Atlantic Deep Water. It is one of the main driving forces of the conveyor belt.

The force of the sinking cold water pushes the North Atlantic Deep Water south. When it reaches Antarctica, the water then flows east. It is pushed along by the Antarctic Circumpolar Current. This current is huge and very strong. It circles the continent of Antartica.

Parts of the Antarctic Circumpolar Current flow north. They then move into the Indian Ocean and Pacific Ocean. As the deep, cold water travels, it mixes with warmer water. Over time the water becomes warm enough to rise. This brings nutrients to the surface.

A 500-Year Process

In the Pacific, the surface water flows into the Indian Ocean. It then travels around southern Africa, and back into the Atlantic. In time, the warm waters travel back to the North Atlantic Deep Water. With that, the circle is completed.

One complete trip around Earth takes 500 years. During that time, all the ocean's water is turned over.