Covering three quarters of the Earth

Covering three quarters of the Earth's surface, oceans are as diverse as they are large. The open seas is an interesting and little understood world full of bizarre and fascinating creatures. This ecosystem is a self-containing world with a complex food chain and many organisms that have adapted to extreme and unusual conditions. The ocean is home to the smallest plankton and the largest creature on earth, the blue whale. Come explore the open seas with us!

Why is the ocean blue?
No one knows for sure. There are several theories as to why the ocean is usually blue. Some scientists believe that when sunlight hits seawater, part of the white light is absorbed and part is reflected. White light is composed of various shades (all the colors in the rainbow). Seawater does not absorb blue light, so the blue light is reflected outwards. Some scientists believe that suspended matter in the sea is responsible for its blue color.

What causes waves?
Wind is the most common cause of surface waves. The longer the distance a wind blows, the stronger it is, and the bigger the waves. As wind blows over the ocean, it tries to pick up some of the water. Because the surface of the water can not move, it rises. Gravity pulls the wave back down into the ocean. However, the water now has momentum. Water pressure from below pushes the water back up, and the cycle starts again.

How far can sunlight penetrate into the ocean?
Significant sunlight can only penetrate 100 meters freely. Because seawater is relatively transparent, about 5% of sunlight penetrates to a depth of 262 feet (80 meters). However, if the water contains significant amounts of silt, algae or other factors, the penetration depth can be reduced to 164 feet (50 meters) or less.

Learn more about the ocean light zone.

Light Zones

The ocean can be divided from its surface to its depth into three zones based on the amount of light received. They are:

Sunlight Zone 1. Sunlit Zone: This is the top layer, nearest the surface. It is also called the euphotic zone. Here there is enough light penetrating the water to support photosynthesis.

Because photosynthesis occurs here, more than 90 percent of all marine life lives in the sunlit zone.

The sunlit zones goes down about 600 feet. Many animals inhabit this zone. Most fish live in this zone. Plankton occurs in this zone. Plankton are free-floating aquatic organisms. They are usually microscopic and form the basis of the food chain in the ocean.

2. Twilight Zone: Only a small amount of light can penetrate the water at this depth. As the water becomes deeper, the pressure increase, too. Plants do not grow here. Only animals that have adapted to little light survive.

The twilight zone is also known as the disphotic zone.

Animals that live in the twilight zone include: lantern fish, rattalk fish, hatchet fish, viperfish, and mid-water jellyfish.

This murky part of the ocean begins at about 600 feet under the water and extends to the darkest part, which begins about 3000 feet down.

Some squid and fish can use their bodies to make light. These creatures are said to have bioluminescence.

Zonation

The ocean can be divided into many zones. The ocean bottom is the benthic zone and the water itself (or the water column) is the pelagic zone. The neritic zone is that part of the pelagic zone that extends from the high tide line to an ocean bottom less than 600 feet deep. Water deeper than 600 feet is called the oceanic zone, which itself is divided on the basis of water depth into the epipelagic, mesopelagic, and bathypelagic zones. These zones roughly correspond to the three other zones divided on the basis of the amount of sunlight they receive. In the sunlit zone, enough light penetrates to support photosynthesis. Below that lies the twilight zone, where very small amounts of light penetrate. Ninety percent of the space in the ocean lies in the midnight zone, which is entirely devoid of light.

(Click on the image to see a larger version.)

How many oceans are there in the world?
There is really only one big ocean. One can sail in a boat in every known "ocean" and "sea", which means they are all connected.

What are the main "oceans" called?
It is generally recognized that there are five main areas in this one big ocean:

The Pacific Ocean
The Atlantic Ocean
The Arctic Ocean surrounds the North Pole. It is the smallest "ocean" with only 5.5 million square miles and 4% of the world's water. It is frozen all the time, except at its edges.
The Indian Ocean is the third largest area. It covers about 28 million square miles and holds 20% of the world's water. About 90% of the Indian Ocean is south of the equator.
The Southern Ocean is where you'll find Antarctica and the South Pole. Some say it is not an "ocean" at all, but the southern part of three "oceans": the Pacific, Atlantic, and Indian.

What is a sea?
A sea is a branch of one of the five "oceans". A sea is partially enclosed by land. For example, the Indian Ocean is divided in two by the country of India, forming the Arabian Sea on one side and the Bay of Bengal on the other.

What are the world's largest seas?

1. The South China Sea
2. The Caribbean Sea
3. The Mediterranean Sea

What are the world's largest islands?

1. Greenland, northern Atlantic
2. New Guinea, southwest Pacific
3. Borneo, southwest Pacific
4. Madagascar, western Indian
5. Baffin Island, northern Atlantic

Why is the ocean salty?
It is salty because it has a lot of salt in it! Salt is a mineral that is found in soil and rocks, too. Flowing water picks up some salt and carries it to rivers. Rivers carry it to seas. Since this has been happening for millions of years, the oceans now have a lot of salt—about one cup per gallon!

Where is the deepest part of the ocean?
The deepest place is near the Mariana Islands in the Pacific. The water is over 36,000 feet deep in the Mariana Trench. That's over 7 miles deep. Mount Everest is only 6 miles high!