Ocean

An ocean is a major division of the continuous body of seawater that covers approximately 71 percent of Earth's surface, constituting the dominant feature of the hydrosphere and one of the most consequential systems for life and climate on the planet. The collective mass of ocean water, sometimes called the world ocean or global ocean, holds an estimated 97 percent of all water on Earth and averages about 3,688 metres in depth, though these figures are subject to ongoing measurement refinement.

Oceans regulate global temperature, drive weather patterns, absorb carbon dioxide, produce roughly half of Earth's atmospheric oxygen through marine photosynthesis, and support an estimated 50 to 80 percent of all life on the planet — though the true extent of marine biodiversity remains an active research frontier.

The five named oceans

Geographers and oceanographers conventionally recognise five oceans, though all are interconnected as a single continuous body of water.

Pacific Ocean

The Pacific Ocean is the largest and deepest ocean, covering more than 165 million square kilometres and containing the deepest known point on Earth — the Mariana Trench, which reaches approximately 10,935 metres below sea level. It spans from the Arctic in the north to the Southern Ocean in the south, and from Asia and Australia in the west to the Americas in the east.

Atlantic Ocean

The Atlantic Ocean is the second largest ocean, covering roughly 106 million square kilometres. It separates the Americas from Europe and Africa and is bisected along much of its length by the Mid-Atlantic Ridge, a major undersea mountain chain and site of significant tectonic activity. The Atlantic plays a crucial role in global thermohaline circulation, including the Gulf Stream.

Indian Ocean

The Indian Ocean covers approximately 70 million square kilometres, bounded by Africa to the west, Asia to the north, Australia to the east, and the Southern Ocean to the south. It is the warmest of the world's oceans and strongly influences the monsoon climate systems of South and Southeast Asia.

Southern Ocean

The Southern Ocean encircles Antarctica and is the youngest of the five oceans in terms of formal recognition; the International Hydrographic Organization officially delimited it in 2000, though its boundaries remain subject to some variation in usage. It is defined by the Antarctic Circumpolar Current, the largest ocean current on Earth.

Arctic Ocean

The Arctic Ocean is the smallest and shallowest of the five, covering roughly 14 million square kilometres. It is largely covered by sea ice for much of the year, though the extent of this ice has declined markedly in recent decades in association with climate change.

Physical properties and structure

Salinity

Seawater contains an average salinity of approximately 3.5 percent by mass, with dissolved salts dominated by sodium chloride. Salinity varies by region: it is higher in enclosed or semi-enclosed seas such as the Mediterranean and lower near river mouths and polar regions where freshwater input is significant.

Temperature and layering

The ocean is thermally stratified into distinct layers. The surface zone (epipelagic zone), extending to roughly 200 metres, receives sunlight and is biologically productive. Below this lies the thermocline, a layer of rapidly decreasing temperature. The deep ocean below approximately 1,000 metres maintains near-freezing temperatures and near-total darkness, yet supports diverse communities of organisms.

Ocean currents

Ocean currents are driven by wind, differences in water density (thermohaline circulation), and Earth's rotation (the Coriolis effect). Surface currents redistribute heat from the tropics toward the poles and have a profound moderating influence on coastal and continental climates. Deep-water currents form part of the global conveyor belt, transporting cold, dense water along the ocean floor on timescales of centuries to millennia.

The ocean and climate

The ocean absorbs a substantial fraction of the excess heat generated by the greenhouse effect, buffering the rate of atmospheric warming. It also absorbs large quantities of carbon dioxide, forming carbonic acid — a process that has measurably lowered ocean pH in a phenomenon known as ocean acidification. Ocean acidification poses documented risks to shell-forming organisms, coral reefs, and broader marine food webs.

Sea surface temperatures influence the formation and intensity of tropical cyclones, and shifts in ocean circulation have been linked to regional drought, flooding, and altered fishery productivity.

Marine ecosystems and biodiversity

The ocean hosts ecosystems ranging from shallow-water coral reefs — among the most biodiverse environments on Earth — to hydrothermal vent communities on the deep seafloor that derive energy from chemical reactions rather than sunlight. Open-ocean (pelagic) ecosystems are dominated by plankton, which form the base of the marine food web and are primary producers of atmospheric oxygen via photosynthesis.

Estimates of total marine species counts vary widely; many deep-sea regions remain incompletely surveyed, and new species are regularly described. Human activities including overfishing, pollution, habitat destruction, and climate change have placed significant pressure on marine biodiversity globally.

Human relationship with the ocean

Humans have used the ocean for food, trade, transportation, and cultural identity throughout recorded history and well before. Maritime trade routes shaped economic and political history from antiquity through the age of European exploration (roughly the 15th to 17th centuries) to the present, when over 80 percent of global trade by volume moves by sea.

The ocean is also a source of oil, natural gas, minerals, and increasingly offshore wind energy. Coastal and island communities worldwide depend on marine fisheries and reef ecosystems for food security and livelihoods.

Legal governance of the ocean is established primarily through the United Nations Convention on the Law of the Sea (UNCLOS), adopted in 1982 and in force since 1994, which defines national maritime zones and the rights and responsibilities of states in ocean use.

Frequently asked questions

Is the ocean one body of water or several?

All of Earth's oceans are physically connected and form a single continuous body of saline water. The division into named oceans — Pacific, Atlantic, Indian, Southern, and Arctic — is a geographical and administrative convention.

Is the ocean the same as the sea?

The terms are related but not identical. Ocean typically refers to the five major named divisions of the world ocean, while sea often denotes a smaller, partially enclosed body of saltwater, such as the Mediterranean Sea or the Caribbean Sea, though usage overlaps in everyday language.

Is most of the ocean unexplored?

The majority of the ocean floor, particularly in the deep sea, has not been directly observed or mapped at high resolution. Estimates commonly cited in oceanography suggest that more than 80 percent of the ocean remains poorly explored, though this figure depends on the definition of explored and is subject to ongoing revision as mapping technology improves.

Is ocean acidification reversible?

Ocean acidification results from the absorption of atmospheric carbon dioxide and is closely tied to emissions levels. While the process could slow or reverse on geological timescales if atmospheric CO₂ concentrations were substantially reduced, on human timescales the changes already underway are considered largely irreversible without significant intervention.

Is the Southern Ocean officially recognised?

The Southern Ocean was officially recognised as the fifth ocean by the International Hydrographic Organization in 2000, though acceptance has not been universal across all national geographic authorities. The U.S. National Geographic Society, for example, formally adopted the designation in 2021.