The five oceans

Look at a globe. Most of it is blue.

All that blue water is joined together. It is one big ocean. People give different parts of it different names.

From space, Earth looks blue because most of it is covered by water. NOAA, the U.S. ocean and weather agency, says the ocean covers about 71 percent of Earth's surface and holds about 97 percent of all Earth's water. Of the rest, about 2 percent is frozen in glaciers and ice caps, and less than 1 percent is liquid fresh water.

All of that seawater is connected. There are no walls between the oceans. So scientists say there is really one global ocean, which people divide into five named oceans to make it easier to talk about different parts of it.

NOAA describes one global ocean covering 71 percent of Earth, geographically divided into named regions whose boundaries 'have evolved over time for a variety of historical, cultural, geographical, and scientific reasons'. National Geographic Education makes the same point: oceanographers divide the world ocean into five principal basins.

The volume is immense. NOAA cites the U.S. Geological Survey's estimate of over 1,386,000,000 cubic kilometers of water on Earth, of which NOAA's National Geophysical Data Center estimates 1,335,000,000 cubic kilometers is in the ocean. NOAA Ocean Exploration gives the ocean's surface area as about 360 million square kilometers and its average depth as 3,682 meters, an estimate made in 2010 by NOAA and Woods Hole Oceanographic Institution scientists using satellite measurements.

For a long time, people named four oceans.

Now most countries name five. The newest name is the Southern Ocean, all the way around Antarctica.

NOAA explains that, historically, there were four named oceans: the Atlantic, Pacific, Indian and Arctic. Today most countries, including the United States, also recognize the Southern Ocean as a fifth.

The Southern Ocean is the newest name. In the United States it is defined as the water from the coast of Antarctica north to the line of latitude at 60 degrees south. Its boundaries were proposed in 2000 to the International Hydrographic Organization, an international group that the United States belongs to. Not every country agrees on them yet.

The count of oceans is a naming decision. NOAA notes four historical oceans (Atlantic, Pacific, Indian, Arctic) and that most countries, including the U.S., now recognize the Southern (Antarctic) Ocean as a fifth. The U.S. Board on Geographic Names defines it as extending from the coast of Antarctica to 60 degrees south.

Those boundaries were proposed to the International Hydrographic Organization (IHO) in 2000, but because not all countries agree, they have not been ratified by IHO members. The U.S. is a member, represented by NOAA's Office of Coast Survey. National Geographic adds that the Antarctic Convergence, where cold Antarctic water meets warmer water, has limited the exchange of marine life since it arose around 20 million years ago.

The Pacific is the biggest ocean.

It is so big that all the continents could fit inside it! Its name means peaceful.

The Pacific is by far the largest ocean. NOAA says it covers about 155 million square kilometers and holds more than half of the free water on Earth. All of the world's continents could fit into it!

It got its name from the explorer Ferdinand Magellan. In November 1520, after sailing through the straits at the tip of South America, his small fleet entered a new ocean. The water was calm, so he called it pacific, which means peaceful.

NOAA gives the Pacific about 155 million km² (59 million square miles) and more than half of Earth's free water, making it by far the largest ocean basin; every continent could fit within it. The deepest known point in the ocean, Challenger Deep in the Mariana Trench, lies beneath the western Pacific at about 10,935 m.

The name dates from Ferdinand Magellan's voyage for Spain. After passing through what is now the Strait of Magellan, his fleet entered the ocean in November 1520, and he called it pacific because the water was calm at the time. NOAA notes the crew believed the Spice Islands were close; their destination was still thousands of miles away.

The Atlantic Ocean is the second biggest.

The Indian Ocean is the third biggest. It lies between Africa, Asia and Australia.

The Atlantic is the second-largest ocean, covering about 106 million square kilometers, NOAA says. That is about 20 percent of Earth's surface, but only slightly more than half the size of the Pacific. It lies between the Americas on the west and Europe and Africa on the east, and connects to the Arctic Ocean in the north and the Southern Ocean in the south.

The Indian Ocean is third, at about 70.6 million square kilometers. It lies between Africa, Asia and Australia.

NOAA puts the Atlantic at approximately 106,460,000 km² (41,105,000 square miles), about 20 percent of Earth's surface and slightly more than half the Pacific's size. Scientists often split it into North and South Atlantic basins. The North Atlantic, where water sinks after being chilled by Arctic temperatures, is the start of the global ocean conveyor, a circulation that helps regulate Earth's climate.

The Indian Ocean, third in size at about 70.6 million km² (Wikidata), is bounded by Africa, southern Asia and Australia. Together, the Pacific, Atlantic and Indian are the three basins NOAA calls the most commonly known.

Two oceans are very cold.

The Southern Ocean goes around Antarctica. The Arctic Ocean is at the top of the world. It is the smallest ocean.

The two smallest oceans surround the poles. The Southern Ocean circles Antarctica and covers about 20 million square kilometers. Huge flat icebergs float in it. Blue whales come here to feed: National Geographic says a grown blue whale can eat four to eight tons of tiny krill a day during the feeding season.

The Arctic Ocean, about 14 million square kilometers, is the smallest. It sits around the North Pole, mostly surrounded by land. NSIDC explains that sea ice grows on it through autumn and winter and melts through spring and summer.

The Southern Ocean, about 20.3 million km² (Wikidata), extends from Antarctica's coast to 60 degrees south under the U.S. definition. National Geographic Explorer-in-Residence Enric Sala notes that a full-grown blue whale can eat four to eight million krill, four to eight tons, a day for six months of the feeding season.

The Arctic Ocean, about 14.1 million km² (Wikidata), is the smallest of the five. NSIDC describes the north polar region as largely an ocean surrounded by land, where sea ice grows through autumn and winter and melts through spring and summer. Its sea ice is part of what makes the Arctic such a distinct region for weather and climate.

Ask a grown-up to help you count 100 spoons of water.

Put 97 spoons in a big bowl: that is the ocean. Put 2 spoons in an ice tray: that is ice. Only 1 spoon is left!

NOAA says about 97 percent of Earth's water is in the ocean, about 2 percent is frozen in glaciers and ice caps, and less than 1 percent is liquid fresh water. You can make a model.

Count 100 spoonfuls of water. Put 97 in a big bowl for the ocean, 2 in an ice cube tray for the ice, and keep the last spoonful in a cup. That last spoonful stands for all the liquid fresh water, and it is even a little too much. Then find all five oceans on a globe, and trace a path through each one without crossing land.

Build a scale model from NOAA's figures. Of 100 units of Earth's water, about 97 are ocean, about 2 are glaciers and ice caps, and less than 1 is liquid fresh water. Then work with NOAA's volumes: if 1,335,000,000 of about 1,386,000,000 cubic kilometers is ocean, what percentage is that? Compare your answer with the 97 percent figure.

Next, use the areas: the Pacific (about 155 million km², NOAA) and the four other basins. Roughly what fraction of NOAA Ocean Exploration's 360 million km² is the Pacific alone? Finally, on a globe, test NOAA's claim that every continent could fit into the Pacific basin by estimating their combined area.