An ocean is one of the five great divisions of the continuous body of salt water that covers about 70.8% of Earth’s surface and holds 97% of the planet’s water. What separates an ocean from a sea, gulf or bay is scale and geology: an ocean fills a whole basin floored by dense, basaltic oceanic crust, created where tectonic plates pull apart, while seas and bays are smaller, partly enclosed margins of that basin. By area the five are the Pacific (about 165.25 million km²), the Atlantic (about 85.1 million km²), the Indian (70.56 million km²), the Southern (21.96 million km²) and the Arctic (14.06 million km²). Their average depth is 3,688 m, and the deepest point, the Challenger Deep in the Pacific’s Mariana Trench, lies about 10,935 m down.
Plenty of names on the map break those rules, so each section below tests the definition against one look-alike.
Ocean Versus Sea: Where the IHO Draws the Line
Used loosely, “the ocean” and “the sea” mean the same thing: the single, interconnected World Ocean. The Russian oceanographer Yuly Shokalsky coined the term World Ocean in the early 20th century to stress that all the basins exchange water freely. In stricter use, a sea is a division of that ocean which is partly or wholly enclosed by land, and it is generally much smaller. A marginal sea such as the North Sea sits on a continental shelf at the edge of an ocean; an inland sea such as the Mediterranean is almost ringed by continents.
The boundaries themselves are a matter of agreement rather than nature. The first International Hydrographic Conference opened in London on 24 July 1919, with hydrographers from 24 nations, and led to the International Hydrographic Bureau, founded in 1921 and now the International Hydrographic Organization (IHO). The bureau published the first edition of Limits of Oceans and Seas in 1928, with further editions in 1937 and 1953. The 1953 third edition is still the one in force, and it is where the line between, say, the Indian Ocean and the Arabian Sea is formally drawn.
Size alone does not separate the two. The Philippine Sea, a marginal sea of the western Pacific and the largest sea in the world, covers an estimated 5 million km², roughly a third of the area of the entire Arctic Ocean. The Mediterranean covers about 2.5 million km², just 0.7% of the global ocean surface. What makes both of them seas rather than oceans is that each is a bounded part of a larger basin, edged by island chains or coastlines.
Five Oceans, One Connected World Ocean
Many older atlases showed four oceans. The five-ocean model only settled in the early 21st century, when the Southern Ocean, defined by the Antarctic Circumpolar Current, was recognised by the U.S. Board on Geographic Names in 1999 and by the IHO in its 2000 draft revision. That revision put the Southern Ocean’s northern limit at 60° south, although it has never been formally adopted because of unrelated disputes, including the naming of the Sea of Japan. The National Geographic Society recognised the Southern Ocean on its maps in June 2021.
The Pacific alone covers about 32% of Earth’s total surface, more than all the land on the planet combined (148.94 million km²), and has a mean depth of 4,000 m. The Atlantic, at about 17% of Earth’s surface, separates the Americas from Europe and Africa. The Indian Ocean is the warmest and geologically the youngest, its basin opening as the ancient Tethys Ocean broke up; its average depth is 3,741 m. The Arctic is the smallest, shallowest and coldest, and is mostly covered by sea ice. It is so fed by rivers and so poorly mixed with the open ocean that some oceanographers call it the Arctic Mediterranean Sea or treat it almost as an estuary of the Atlantic.
The Sargasso Sea: A Sea With No Coastline
If seas are defined by the land around them, the Sargasso Sea shows how loose the vocabulary is: it is the only named sea with no land boundary at all. It lies in the North Atlantic, between about 20° and 35° north and 40° and 70° west, and is bounded entirely by currents: the Gulf Stream to the west, the North Atlantic Current to the north, the Canary Current to the east and the North Atlantic Equatorial Current to the south. Together those four form the clockwise North Atlantic Gyre.
The sea is about 1,100 km wide and 3,200 km long, with Bermuda near its western fringe. Portuguese navigators named it after the floating Sargassum seaweed (sargaço in Portuguese), and Christopher Columbus recorded his fear in 1492 that the weed might trap his ships. Its water is unusually clear, with visibility down to about 60 m.
Caspian and Dead Sea: “Seas” That Are Really Lakes
The opposite mistake also appears on maps: bodies of water called seas that have no link to the World Ocean at all. The Caspian Sea is the world’s largest inland body of water, covering 371,000 km² between Kazakhstan, Russia, Azerbaijan, Iran and Turkmenistan. It sits in an endorheic basin, with no outflow, and its surface lies about 27 m below sea level. Its salinity is about 1.2%, roughly a third of ordinary seawater’s 3.5%, and its main inflow is the Volga. Hydrologically it is a lake, even though its size and salt water earned it the name sea.
The Dead Sea is smaller but more extreme. It is a landlocked salt lake bordered by Jordan, the West Bank and Israel, fed by the Jordan River. In 2025 its surface stood 439.78 m below sea level, making its shores the lowest land on Earth, and with a salinity of 34.2% (measured in 2011) it is 9.6 times as salty as the ocean. The Dead Sea lies in the Jordan Rift Valley, a basin formed along the Dead Sea Transform fault between the African and Arabian plates. The Caspian is more surprising: its South Caspian Basin, a remnant of the ancient Paratethys Sea, is floored by oceanic basalt, yet it has been landlocked for about 5.5 million years. Being cut off from the World Ocean is what makes it a lake rather than part of an ocean.
Bay of Bengal, Gulf of Mexico, Hudson Bay: Bigger Than Many Seas
Bays and gulfs are recessed bodies of water opening onto a larger one. In principle a gulf is bigger than a bay, but names follow history, not size. The Bay of Bengal, the north-eastern arm of the Indian Ocean between India, Bangladesh, Myanmar and the Andaman and Nicobar Islands, covers 2.6 million km². That is larger than the Mediterranean. Rivers including the Ganges, Brahmaputra and Irrawaddy drain into it, and its coast holds the Sundarbans, the largest mangrove forest in the world.
The Gulf of Mexico, a marginal sea of the Atlantic, covers about 1.6 million km² and is roughly 1,500 km wide. Almost half of it is shallow continental shelf, and its narrow links to the Atlantic, through the Straits of Florida and the Yucatán Channel, keep its tides very small. Hudson Bay in Canada covers 1.23 million km² and is classed as an inland marginal sea of the Arctic Ocean, joined to the Atlantic by the Hudson Strait. All three are bigger than many water bodies called seas, and the main thing separating them from an ocean is that each is enclosed on most sides by land. For a closer look at how these inlets are classified, see our guide to bays, gulfs and fjords.
Oceanic Crust and the Red Sea Rift: How an Ocean Is Born
The deepest difference between an ocean and its look-alikes lies underneath. Oceans sit on oceanic crust, made mainly of dark, iron- and magnesium-rich rocks such as basalt and gabbro. It is usually less than 10 km thick, against 25 to 70 km for continental crust, and denser, at about 3.0 g/cm³ compared with about 2.7 g/cm³ for continental crust. Because it is heavier, it rides lower on the mantle, and the basins it forms fill with water. New oceanic crust is made at spreading centres, where magma rises between diverging plates and cools into pillow lavas.
The Red Sea shows this process at an early stage. It is a narrow sea of about 438,000 km² between Africa and the Arabian Peninsula, linked to the Indian Ocean through the Bab-el-Mandeb Strait. Underneath it runs the Red Sea Rift, a spreading ridge between the African and Arabian plates that has changed from a continental rift into an oceanic one. Magnetic stripes on its floor suggest spreading of about 1 cm a year on each side of the rift. If that continues for tens of millions of years, the Red Sea could widen into a true ocean, as the Atlantic did after the Americas split from Europe and Africa.
Depth Compared: From the Challenger Deep to the Molloy Deep
Depth is the last test. About 66% of Earth’s surface is covered by water deeper than 200 m, and nearly half of all marine water lies below 3,000 m. Only oceans reach these depths, in trenches where one plate sinks beneath another. Between December 2018 and August 2019 the American explorer Victor Vescovo’s Five Deeps Expedition became the first to visit the deepest point of every ocean in a crewed submersible, the Limiting Factor.
The Challenger Deep, at the southern end of the Mariana Trench, is the deepest known point on Earth; a 2021 study put it at 10,935 m, give or take 6 m. The Atlantic bottoms out at 8,376 m in the Puerto Rico Trench, which Vescovo reached on 19 December 2018. The Southern Ocean’s deepest point, 7,434 m in the South Sandwich Trench, was mapped in February 2019. The Sunda Trench off Java and Sumatra reaches 7,290 m, and the Arctic’s Molloy Deep in the Fram Strait reaches 5,550 m. By contrast the Red Sea’s deepest point is 2,730 m, and the Caspian’s southern basin reaches 1,023 m below sea level. For more on these trenches, read our list of the deepest ocean trenches in the world.
Put together, the comparisons give a working definition. An ocean is a full basin of the connected World Ocean, floored by oceanic crust, millions of square kilometres in area and thousands of metres deep. A sea is a smaller, partly enclosed part of that basin. A gulf or bay is an inlet named by custom. A “sea” with no outlet, like the Caspian, is really a lake. To see how the Atlantic compares with the others in more detail, continue with our guide to the Atlantic Ocean.