Seas and Oceans: Definitions, Features and the Main Ones | Sea Life, Islands and Oceania

SEAS AND OCEANS

Ocean zones
Many people use the terms "ocean" and "sea" interchangeably when speaking about the ocean, but there is a difference between the two in the eyes of geographers. Seas are smaller than oceans and are usually located where the land and ocean meet. Typically, seas are found on the margins of the ocean and are partially enclosed by land. The Bering Sea, for example, is part of the Pacific Ocean. The Sargasso Sea is an exception. It is defined only by ocean currents. [Source: NOAA]

According to the “Introduction to Physical Oceanography”: There is only one ocean. It is divided into three named parts by international agreement: the Atlantic, Pacific, and Indian oceans (International Hydrographic Bureau, 1953). Seas, which are part of the ocean, are defined in several ways. [Source: Robert Stewart, “Introduction to Physical Oceanography”, Texas A&M University, 2008]

The Pacific Ocean is the largest and deepest of the world's ocean basin The Atlantic basin is the second largest basin, followed by the Indian Ocean basin, the Southern Ocean, and finally the Arctic Ocean basin. The largest ocean that ever existed on our planet was Panthalassa, a world-spanning sea that surrounded the supercontinent Pangaea from about 300 million to 200 million years ago. The Pacific covers more than 30 percent of the planet's surface and stretches 19,000 kilometers (12,000 miles) at its widest point, between Colombia and the Malay Peninsula.

Websites and Resources: National Oceanic and Atmospheric Administration (NOAA) noaa.gov; “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 uv.es/hegigui/Kasper ; Woods Hole Oceanographic Institute whoi.edu ; Cousteau Society cousteau.org ; Monterey Bay Aquarium montereybayaquarium.org

Defining Oceans and Seas

A sea is defined as a division of the ocean which is enclosed or partially enclosed by land. With that said, the Caspian Sea, Dead Sea, and Aral Sea are actually saltwater lakes, because they lack an outlet to the ocean. Conversely, by this definition, the Gulf of Mexico and Hudson Bay are seas. Seas are mostly surrounded by land. By this definition, the Arctic, Mediterranean and Caribbean Seas are clearly seas. Marginal Seas are defined by only an indentation in the coast. The Arabian Sea and South China Sea are marginal seas.

The largest sea is the Bering Sea at 2,270,000 square kilometers (876,000 square miles). The saltiest and warmest sea in the world is the Red Sea with 41 parts of salt per 1,000 parts of water and temperatures ranging from 68 degrees to 87.8 degrees F depending upon which part you measure. The coldest seas are found near the poles such as the Greenland, Barents, Beaufort, Kara, Laptev and East Siberian Seas found near the north pole and Weddell and Ross Seas found in the south poles. The Baltic Sea is also considered one of the coldest seas. Depending upon the amount of salt in the water, sea water freezes at about 28 degrees F. High salt content lowers the temperature for freezing and low salt content raises the temperature for freezing. [Source: Library of Congress]

Robert Stewart wrote in the “Introduction to Physical Oceanography”: The Pacific Ocean extends northward from Antarctica to the Bering Strait. The boundary between the Pacific and Indian Ocean follows the line from the Malay Peninsula through Sumatra, Java, Timor, Australia at Cape Londonderry, and Tasmania. From Tasmania to Antarctica it is the meridian of South East Cape on Tasmania 147◦E. The Indian Ocean extends from Antarctica to the continent of Asia including the Red Sea and Persian Gulf. The boundary between the Atlantic and Pacific is the line forming the shortest distance from Cape Horn to the South Shetland Islands. In the north, the Arctic Sea is part of the Atlantic Ocean, and the Bering Strait is the boundary between the Atlantic and Pacific. [Source: Robert Stewart, “Introduction to Physical Oceanography”, Texas A&M University, 2008]

The Southern Ocean is the 'newest' named ocean. It is recognized by the U.S. Board on Geographic Names as the body of water extending from the coast of Antarctica to the line of latitude at 60 degrees South.

Panthalassa — the World’s All Time Largest Ocean

Panthalassa was the vast global ocean that surrounded the supercontinent Pangaea from about 300 to 200 million years ago. Because a single supercontinent leaves only one major body of water around it, Panthalassa was far larger than any modern ocean and covered about 70% of Earth’s surface—much larger than today’s Pacific. [Source: Michael Dhar, Live Science, August 13, 2022]

Geologists estimate Panthalassa was at least 3,000 kilometers (1,860 miles) wider than the Pacific is now. Crossing it by jet would have taken roughly 15 hours, compared with 10 across today’s Pacific. Its breakup began when the Atlantic Ocean opened, shrinking Panthalassa and leaving the modern Pacific as its remnant.

Earlier in Earth’s history, when continents had not yet formed, the planet may have been covered entirely by one global ocean. Even now, Earth’s connected waters are often described as a single “world ocean.”

Since Pangaea’s breakup 200 million years ago, the Pacific has been the largest ocean—but that may change. Over the next 70 million years, tectonic movement is expected to split the Pacific in two as Australia moves northward, while the Atlantic continues to widen and eventually becomes the world’s largest ocean.

Seven Seas

Seven Seas of the Ancient World: 1) The Mediterranean Sea; 2) the Adriatic Sea; 3) the Red Sea; 4) the Black Sea; 5) the Caspian Sea; 6) the Persian Gulf; and 7) the Arabian Sea

The exact origin of the phrase 'Seven Seas' is uncertain, although there are references in ancient literature that date back thousands of years. In various cultures at different times in history, the Seven Seas has referred to bodies of water along trade routes, regional bodies of water, or exotic and far-away bodies of water. [Source: NOAA]

What are the seven seas? According to the Library of Congress: There is no definitive answer to this question. The phrase is a figure of speech and has been used to refer to different bodies of water at various times and places. Some ancient civilizations used the phrase “seven seas” to describe the bodies of water known at that time. The ancient Romans called the lagoons separated from the open sea near Venice the septem maria or seven seas. According to NOAA: In Greek literature (which is where the phrase entered Western literature), the Seven Seas were the Aegean, Adriatic, Mediterranean, Black, Red, and Caspian seas, with the Persian Gulf thrown in as a "sea." Most current sources state that “seven seas” referred to the Indian Ocean, Black Sea, Caspian Sea, Adriatic Sea, Persian Gulf, Mediterranean Sea, and the Red Sea.

Not all geographers agree on this list of seven, believing that the seven seas reference will be different depending upon the part of the world and the time period in question. Some geographers point to the Age of Discovery and suggest that the seven seas represent the Atlantic, Pacific, Arctic, and Indian Oceans, as well as the Mediterranean Sea, the Caribbean, and the Gulf of Mexico. Other geographers state that the seven seas were the Mediterranean and Red Seas, Indian Ocean, Persian Gulf, China Sea, and the West and East African Seas. In Medieval European literature, the Seven Seas phrase referred to the North Sea, Baltic, Atlantic, Mediterranean, Black, Red, and Arabian seas. As trade picked up across the Atlantic, the concept of the Seven Seas changed again. Mariners then referred to the Seven Seas as the Arctic, the Atlantic, the Indian, the Pacific, the Mediterranean, the Caribbean, and the Gulf of Mexico.

Not many people use this phrase today, but you could say that the modern Seven Seas include the Arctic, North Atlantic, South Atlantic, North Pacific, South Pacific, Indian, and Southern Oceans. However, our ocean is more commonly geographically divided into the Atlantic, Pacific, Indian, Arctic, and Southern (Antarctic).

Black Sea

Black Sea is a huge inland sea, covering 507,900 square kilometers (196,100 square miles) with an average depth of 3,906 feet. Twice the size of Michigan, it is surrounded by Bulgaria, Romania, Russia, the Ukraine, Georgia and Turkey and is connected to the Mediterranean Sea by the half-mile-wide Bosporus at Istanbul, the Sea of Marmara and the Dardanelles. Because it has few outlets it has virtually no tides. The water is warm enough for swimming in the summer, and can generate some surprisingly large waves in some places when the wind kicks up.

The Black Sea gets it name from storms that frequently darken the skies over the sea and the sea itself. Black Sea generates a lot its own moisture and winds that blow in from the northwest bring even more. The coastal areas of the Black Sea receive quite a bit of rainfall (mostly in the winter) and are quite lush and green. There summertime temperatures are generally significantly cooler than the Mediterranean Sea but still plenty warm enough for topless sunbathing.

Mediterranean Sea

Mediterranean Sea means "Sea in the Middle of the Earth." Covering 2,500,000 square kilometers (970,000 square miles), it stretches 3,700 kilometers (2,300 miles) from the Atlantic Ocean in east to Israel in the west, and is 1,100 kilometers (700 miles) at its broadest point (between Croatia to Libya). They Tyrrrhenian Sea, Adriatic Sea and Aegean Sea are all branches of the Mediterranean, which at one time was a lake.

The Mediterranean became a sea several million years ago when the Atlantic ocean broke through a limestone formation that stretched from Iberia (Spain and Portugal) into Africa, releasing a 1,500-meter-high (5,000-foot-high) series of cascades that transformed southern Europe and filled in the Spain basin over a period of about a 100 years. Today the Mediterranean lies in a very geologically-active earthquake-prone area region, as the people from Turkey and Italy will tell you, made up of the colliding African and Eurasian tectonic plates.

Mediterranean Sea and Black Sea

The surface currents of the Mediterranean move at about 5 kph (three miles per hour) from west east. About 75 meters (250 feet) below the surface is a current that rushes in the opposite direction towards the Atlantic. Relatively few river flow into the Mediterranean and the hot climate of the region produces a high rate of evaporation which means the sea is getting saltier. Without the movement of water between the Mediterranean and the Atlantic, the Mediterranean would be little more than a lake.

The Mediterranean is over 4,875 meters (16,000 feet) deep in a trench east of Crete but has virtually no tides. Water levels are often determined less by tides than winds such as the “Vendaval” (astrong westerly that blows through the Straits of Gibraltar), the “La Tramontana” (the strong wind off the Spanish coast), the “le Mistral” (the cold dry wind northwest wind of the Riviera), the “Khamsin”, the “Sirroco”, the “Gregale” (a northeast wind that blows through Malta) and the Levanter (an eastern wind that brings humidity).

In the 6th century B.C., the Mediterranean was 60 centimeters (two feet) lower than it is today. Over the centuries the Mediterranean has been controlled by Phoenicians, Greeks, Romans, Vandals, Byzantines, Moors, Muslims, Normans, French, Spanish, Turks and English. The English writer Dr. Samuel Johnson once said, "The grand object of traveling is to see the shores of the Mediterranean. On these shores were the great Empires of the world: the Assyrian, the Persian, the Grecian, and the Roman. All our religion, almost all our law, almost all our arts, almost all that sets us above savages, has come to us from the shores of the Mediterranean."

The Mediterranean is surrounded by 27 nations with a total population of almost a billion people and over a hundred different ethnic groups who speak more than 50 languages. The largest islands are Sicily Sardinia, Corsica, Cyprus, Crete and Majorca.

The Mediterranean coast boasts well-preserved ancient cities, beach resorts, huge cliffs, spectacular mountains, pebble beaches, sand beaches, translucent waters that vary from purple to green. Along the windy roads that hug the coast you can find pine forests, olive grove, pistachio trees, orange orchards and even a few banana plantations. The most accessible and beautiful parts of the region are in the west near the Aegean Sea.

The Mediterranean is arguable the world’s biggest tourist draw. Possibly more people enjoy vacationing on its shores than either the Atlantic or the Pacific. The main thing that has drawn tourist over the years is the excellent climate. The swimming season extends from April to November and the air temperatures are mild the remaining months of the year.

Some 13,000 plant species are unique to the Mediterranean, which is regarded as a biodiversity hot spot. In the waters are stripped dolphins, monk seals loggerhead turtles and even sperm whales and few great white sharks. Each year there is great migration of bluefin tuna through the Straits of Gibraltar from the Atlantic to the Mediterranean. With the exception of the tuna, the fisheries in the Mediterranean yield much less fish than the an equivalent area of the Atlantic.

South China Sea

The South China Sea lies south of China’s Guangdong Province and Hong Kong. By some reckonings it is the world's largest sea, covering 3,500,000 square kilometers (1,400,000 square miles). A marginal sea that is part of the Pacific Ocean, the South China Sea stretches from the Karimata and Malacca straits to the Strait of Taiwan and carries tremendous strategic importance.

In the last 2,500 years mariners for Malaysia, China and Indonesia navigated the South China Sea to trade sandalwood, silk, tea and spices. Today it carries roughly a third of the world's shipping and accounts for a tenth of the world's fish catch. China, Vietnam, Taiwan, Malaysia, Indonesia, Brunei and the Philippines all have 200-mile coastal economic zones in the South China Sea. All of these countries also claim the Spratly Islands which are in the middle of the sea

About $5.3 trillion of global trade passes through the South China Sea each year, $1.2 trillion of which passes through U.S. ports. Below the South China Sea is an estimated $3 trillion worth of oil, gas and minerals. Fisheries in the South China Sea have been decimated by overfishing and polluting chemicals from shrimp farms and factories. By some estimates there is enough oil under the South China Sea to last China for 60 years.

The South China Sea is south of China; east of Vietnam; west of the Philippines; east of the Malay Peninsula and Sumatra, up to the Strait of Singapore in the western, and north of the Bangka Belitung Islands and Borneo. In recent years, China’s claim that the entire sea is it exclusive possession has been a hot international issue and a point of outrage with China.

Sargasso Sea

The Sargasso Sea, located entirely within the Atlantic Ocean, is the only sea without a land boundary. While all other seas in the world are defined at least in part by land boundaries, the Sargasso Sea is defined only by ocean currents. It lies within the Northern Atlantic Subtropical Gyre. The Gulf Stream establishes the Sargasso Sea's western boundary, while the Sea is further defined to the north by the North Atlantic Current, to the east by the Canary Current, and to the south by the North Atlantic Equatorial Current. Since this area is defined by boundary currents, its borders are dynamic, correlating roughly with the Azores High Pressure Center for any particular season.

The Sargasso Sea is a vast patch of ocean named for a genus of free-floating seaweed called Sargassum. While there are many different types of algae found floating in the ocean all around world, the Sargasso Sea is unique in that it harbors species of sargassum that are 'holopelagic' — this means that the algae not only freely floats around the ocean, but it reproduces vegetatively on the high seas. Other seaweeds reproduce and begin life on the floor of the ocean.

healthy sargassum ecosystem

Sargassum provides a home to an amazing variety of marine species. Turtles use sargassum mats as nurseries where hatchlings have food and shelter. Sargassum also provides essential habitat for shrimp, crab, fish, and other marine species that have adapted specifically to this floating algae. The Sargasso Sea is a spawning site for threatened and endangered eels, as well as white marlin, porbeagle shark, and dolphinfish. Humpback whales annually migrate through the Sargasso Sea. Commercial fish, such as tuna, and birds also migrate through the Sargasso Sea and depend on it for food.

Image Sources: Wikimedia Commons; YouTube, NOAA

Text Sources: National Oceanic and Atmospheric Administration (NOAA) noaa.gov; “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 uv.es/hegigui/Kasper ; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated November 2025