World Oceans and Major Seas

The world ocean is the continuous body of salt water that covers about 71 percent of Earth’s surface and is divided by geographers into five named oceans, the Pacific, Atlantic, Indian, Southern and Arctic, with smaller regions called seas along their margins. In brief: the Pacific is the largest and deepest ocean (about 46 percent of the ocean area), the Atlantic is second, and the Arctic is the smallest and shallowest. The average depth of the ocean is about 3,700 m. Seas such as the Mediterranean, Caribbean and South China Sea are parts of the oceans partly enclosed by land.

The oceans regulate climate, hold nearly all of Earth’s water and carry shipping, fishing and trade. This guide describes the five oceans and the main seas, compares the oceans in a table, works through calculations of area and volume, outlines the history of ocean naming and exploration and corrects common mistakes.

What is the world ocean?

All the oceans are connected, so oceanographers speak of a single world ocean. It holds about 97 percent of Earth’s water and has an average salinity of about 35 parts per thousand (3.5 percent). The sea floor has continental shelves (shallow margins), slopes, wide abyssal plains about 4,000–6,000 m deep, the mid-ocean ridges (the longest mountain chain on Earth) and deep trenches. The separation into five oceans is a convention based on continents and currents. The ocean absorbs and moves heat through currents, so it moderates the climate, produces much of the oxygen in the air and takes up a large share of the carbon dioxide released by human activity.

What are the five oceans?

• Pacific Ocean: the largest and deepest, between Asia and Australia in the west and the Americas in the east; it contains the Mariana Trench with the Challenger Deep (about 10,900 m) and the Ring of Fire, a belt of volcanoes and earthquakes around its edges.
• Atlantic Ocean: an S-shaped ocean between the Americas and Europe/Africa, widening and narrowing; it has the Mid-Atlantic Ridge at its center and carries heavy shipping traffic; the Gulf Stream moves warm water north.
• Indian Ocean: mostly in the Southern Hemisphere, bordered by Africa, Asia and Australia; it has monsoon-driven currents that reverse seasonally.
• Southern Ocean: surrounds Antarctica, and is increasingly recognized as a separate ocean, though its northern boundary is set by the Antarctic Circumpolar Current rather than by a continent, and not all countries or sources use it; its sea ice changes with the seasons.
• Arctic Ocean: the smallest and shallowest, around the North Pole, mostly covered by sea ice for much of the year, though the ice has declined in recent decades.

What are the major seas?

A sea is a part of the ocean partly enclosed by land, or a large inland salt lake with the name “sea”. Examples: the Mediterranean Sea (between Europe, Africa and Asia), the Caribbean Sea, the South China Sea, the Arabian Sea, the Bering Sea, the North Sea, the Baltic Sea, the Red Sea, the Sea of Japan, the Coral Sea, the Black Sea and the Caspian Sea (technically the world’s largest lake). Seas are usually shallower than the open ocean, and many are important for fishing, oil and gas, and shipping routes. Narrow connections, called straits (such as Gibraltar and Bosphorus), link seas and oceans.

Oceans compared

Worked example: area and volume

1. Total ocean area: 71 percent of 510 million km² is 0.71 × 510 = 362 million km².
2. Area of the Pacific: 46 percent of 362 = 0.46 × 362 = 166.5 million km² (about 166 million).
3. Pacific compared with the Atlantic: 46 ÷ 23 = 2, so the Pacific is about twice as large as the Atlantic, and 166.5 ÷ 14.5 = 11.5 times as large as the Arctic.
4. Volume of the ocean: with a mean depth of about 3.7 km, volume = 362,000,000 km² × 3.7 km = 1,339,000,000 km³, or about 1.34 billion km³.
5. Pacific volume: if the mean depth of the Pacific is about 4.0 km, its volume is 166.5 million × 4.0 = 666 million km³, about half of the total volume, a larger share than its 46 percent of the area, because it is deeper than average.
6. Salt in the ocean: at 35 g of salt per kilogram of water, and 1 km³ of seawater having a mass of about 1.03 × 10¹² kg, 1 km³ contains roughly 36 million tonnes of salt (35 × 1.03 × 10¹² g ≈ 3.6 × 10¹³ g = 36 million tonnes).

How were the oceans explored and named?

Early peoples, such as the Polynesians, crossed the Pacific in canoes using knowledge of stars and waves. The names come from Greek and European tradition: “Atlantic” from the Atlas mountains, and “Pacific” (peaceful) from Ferdinand Magellan’s expedition in 1520–1521. The HMS Challenger expedition (1872–1876) was the first major scientific survey of the deep ocean and made the first soundings in the Mariana Trench. In the 20th century, sonar mapped the sea floor and led to the theory of plate tectonics, and in 1960 the bathyscaphe Trieste descended to the bottom of the Challenger Deep. The Southern Ocean was delimited in an International Hydrographic Organization proposal in 2000, but its status as a separate ocean is still not accepted in every source.

Which mistakes are common?

Mistake 1: Thinking the oceans are separate

They are connected and exchange water through currents. The five names are a convention.

Mistake 2: Mixing up seas and oceans

Seas are smaller and partly enclosed by land. Some “seas” are lakes (such as the Caspian), and the Sargasso Sea has no land boundaries at all.

Mistake 3: Believing that the Atlantic is the largest ocean

The Pacific is about twice as large as the Atlantic.

Mistake 4: Assuming the ocean floor is flat

It has mountains, ridges, plains and trenches, and some mountains are taller than Everest from base to top.