Article Summary
What this article covers: The five basins of Earth’s oceans, how deep they go, how they move heat around the planet, and the sea stories told about them, using Ireland’s island coastline as a reference point.
Who this is for: Students, curious travellers and anyone planning a coastal trip who wants the key facts about the World Ocean in one place.
Key takeaway: Earth has one continuous ocean covering about 71% of the planet. The Pacific, Atlantic, Indian, Southern and Arctic are named regions of that single body of water.
Expert insight: “On a grey afternoon at Slea Head on the Dingle Peninsula, I watched a swell that had crossed the whole Atlantic break on the rocks below me. The Blaskets were half hidden in spray, and there was nothing between me and the next continent except moving water. You can read that the ocean covers seventy-one per cent of the planet, but the figure only lands when you stand on the edge of it. After that day I stopped thinking of any coastline as separate from the others.” , Ciaran Connolly, Founder of ConnollyCove
Quick facts:
- The ocean holds about 97% of Earth’s water and covers roughly 71% of the planet’s surface, according to NOAA.
- The Pacific Ocean alone covers more area than all of Earth’s land put together.
- The latest Seabed 2030 update puts the share of the seafloor mapped at modern resolution at 28.7%.
Table of Contents
Stand on a headland in Kerry or Donegal and the water in front of you runs, without a single break, all the way to Antarctica. Earth’s oceans are one connected body of salt water, and the names on the map work more like postcodes than walls. That idea sounds abstract until you live on an island. Ireland sits on the eastern rim of the North Atlantic, and almost every story about the island comes back, sooner or later, to the water around it.
This guide sets out the facts about Earth’s oceans first: how many oceans there are, how big and how deep each one is, and why oceanographers now count five. Then it turns to what the encyclopaedias leave out. That includes how the Atlantic affects Irish weather (less simply than the popular story claims) and the sea gods and voyage tales that coastal peoples built around the water.
ConnollyCove is an Ireland-based travel and culture platform, and our interest in the ocean starts on the shore. The questions readers send us about the sea are usually the same ones Google shows in its “People also ask” box. The answers below are drawn from NOAA, the International Hydrographic Organization, Seabed 2030 and peer-reviewed climate research.
What are Earth’s oceans?
Earth’s oceans are a single, continuous World Ocean that covers about 71% of the planet and holds around 97% of its water. Geographers divide it into five named basins: the Pacific, Atlantic, Indian, Southern and Arctic. The boundaries follow continents, ridges and currents, but the water itself moves freely between them. NOAA’s explainer on how many oceans there are puts it plainly: there is one global ocean, and the divisions are a matter of convention.
The total volume is roughly 1.3 billion cubic kilometres. The average depth is a little under 3,700 metres, which is deeper than most of the Alps are high. Seawater averages about 35 grams of dissolved salt per kilogram, though the figure drops near melting ice and river mouths.
Why the Southern Ocean counts as an ocean
The Southern Ocean is defined by a current, not by land. The Antarctic Circumpolar Current flows west to east all the way around Antarctica, and it moves more water than any other current on the planet. That flow keeps the water south of roughly 60°S colder and less salty than the Atlantic, Pacific and Indian waters to its north.
Recognition came in stages. The US Board on Geographic Names adopted the name in 1999, and boundaries were proposed to the International Hydrographic Organization (IHO) in 2000. Not every IHO member agreed, so those boundaries have never been formally ratified. National Geographic began counting the Southern Ocean as the fifth ocean on World Oceans Day, 8 June 2021, and most atlases now list it as the newest of Earth’s oceans.
Five oceans or seven seas?
Five is the modern answer, and “the seven seas” is an older figure of speech. The phrase appears in writing from many cultures and has pointed to different bodies of water over the centuries. A 19th-century convention reached seven by splitting two oceans in half: the North and South Atlantic, the North and South Pacific, plus the Arctic, Indian and Southern Oceans. You will still hear sailors use it, but no scientific body divides Earth’s oceans into seven.
Earth’s five oceans ranked by size
Ranked by surface area, Earth’s oceans run from the Pacific down to the Arctic. The Pacific is almost twice the size of the Atlantic, and the Arctic is roughly a tenth the size of the Pacific. Here is the order, followed by the key figures in one table.
- Pacific Ocean: about 168.7 million km²
- Atlantic Ocean: about 85.1 million km²
- Indian Ocean: about 70.6 million km²
- Southern Ocean: about 22.0 million km²
- Arctic Ocean: about 15.6 million km²
| Ocean | Area (million km²) | Area (million sq mi) | Share of World Ocean | Average depth | Deepest point |
|---|---|---|---|---|---|
| Pacific | 168.7 | 65.1 | 46.6% | about 4,080 m | Challenger Deep, Mariana Trench, about 10,935 m |
| Atlantic | 85.1 | 32.9 | 23.5% | about 3,650 m | Milwaukee Deep, Puerto Rico Trench, about 8,380 m |
| Indian | 70.6 | 27.2 | 19.5% | about 3,740 m | Java (Sunda) Trench, about 7,200 m |
| Southern | 22.0 | 8.5 | 6.1% | about 3,270 m | South Sandwich Trench, about 7,430 m |
| Arctic | 15.6 | 6.0 | 4.3% | about 1,200 m | Molloy Deep, Fram Strait, about 5,550 m |
Area figures follow the CIA World Factbook. Trench depths come from modern sonar surveys and vary by a few tens of metres between sources.
1. The Pacific Ocean: the largest basin
The Pacific covers more area than every continent combined. Its western edge holds the Mariana Trench, where Challenger Deep reaches close to 11 kilometres. Drop Mount Everest in there and its summit would still sit more than two kilometres below the surface. The pressure at the bottom is more than a thousand times what you feel at sea level.
The ocean’s rim is lined with volcanoes and trenches where tectonic plates collide, the zone known as the Ring of Fire. It is also the ocean where people first crossed vast distances without instruments. Polynesian wayfinders read stars, swells and bird flight to settle islands spread across thousands of kilometres. ConnollyCove’s guide to how Polynesian navigators discovered the Pacific covers those techniques in detail.
The Pacific holds the largest coral reef system on Earth as well. The reef in ConnollyCove’s Great Barrier Reef guide stretches for more than 2,000 kilometres along Australia’s north-east coast.
2. The Atlantic Ocean: Ireland’s ocean
The Atlantic is the youngest of the large basins and it is still getting wider. The Mid-Atlantic Ridge runs down its centre like a seam, and the seafloor spreads apart along it by a few centimetres a year. Iceland sits directly on the ridge, which is why it has so many active volcanoes.
That opening shaped Ireland directly. When the North Atlantic began to split open around 60 million years ago, lava poured across what is now County Antrim. The hexagonal basalt columns described in ConnollyCove’s guide to the Giant’s Causeway are one result of that volcanic phase.
The Atlantic’s deepest point is the Milwaukee Deep in the Puerto Rico Trench, at roughly 8,380 metres. Closer to home, the seabed west of Ireland drops away into features like the Porcupine Seabight and the Rockall Trough. Cold-water coral reefs grow on mounds in these areas, far below the reach of sunlight.
For travellers, the Atlantic is the ocean you meet face to face on the island’s western seaboard. ConnollyCove’s Wild Atlantic Way guide follows the coastal route from Donegal to Cork, where the swell arrives after crossing the full width of the basin.
3. The Indian Ocean: the monsoon ocean
The Indian Ocean is the warmest of the five, and it is the only one hemmed in by land to the north. That shape drives the monsoon. Winds blow from the south-west in summer and from the north-east in winter, and for centuries sailors timed their voyages to that reversal.
Arab, Persian, Indian and Swahili traders crossed between East Africa, Arabia and South Asia long before European ships arrived. Their routes carried cloves, nutmeg, cotton and ceramics, a story told in our guide to the spice routes.
The same seasonal winds made the whole system work. Ships sailed east on one monsoon and waited months to ride the reverse wind home, so ports along the route became meeting places for traders from across the ocean. ConnollyCove’s piece on historical trade winds and global trade explains how those wind belts shaped sea routes worldwide.
4. The Southern Ocean: the ring around Antarctica
The Southern Ocean is the stormiest of the five and the only one that circles the globe without meeting land. The Antarctic Circumpolar Current can run unbroken because there is no continent in its way. Where its cold water meets warmer water to the north, at a boundary called the Antarctic Convergence, the temperature and the marine life change within a short distance.
This is where Antarctic krill swarm in vast numbers, feeding whales, seals and penguins. The current also links the Atlantic, Pacific and Indian Oceans, passing heat and carbon between them.
5. The Arctic Ocean: the smallest and shallowest
The Arctic is the smallest ocean and by far the shallowest, with an average depth of about 1,200 metres. Wide continental shelves surround it, and sea ice covers much of the surface for most of the year. Large rivers in Siberia and Canada pour fresh water into it, which gives it the lowest salinity of the five.
Its deepest point, the Molloy Deep, lies in the Fram Strait between Greenland and Svalbard. Summer sea ice has shrunk sharply since satellite records began in the late 1970s. That loss has opened seasonal shipping routes that were once impassable, along with new questions about who controls them.
How deep the ocean goes: the five depth zones
Oceanographers divide the water column of Earth’s oceans into five layers based on how much sunlight reaches them. Sunlight fades fast in seawater. Below about 200 metres there is too little light for photosynthesis, and below 1,000 metres there is none at all apart from the glow that some animals make themselves.
| Zone | Depth range | Light | What lives there |
|---|---|---|---|
| Epipelagic (sunlight zone) | 0 to 200 m | Enough for photosynthesis | Phytoplankton, most fish, dolphins, sea turtles |
| Mesopelagic (twilight zone) | 200 to 1,000 m | Faint, fading to none | Lanternfish, squid, animals that migrate upward at night |
| Bathypelagic (midnight zone) | 1,000 to 4,000 m | None, apart from bioluminescence | Anglerfish, gulper eels, sperm whales hunting squid |
| Abyssopelagic (abyss) | 4,000 to 6,000 m | None | Sea cucumbers, tripod fish, scavenging amphipods |
| Hadalpelagic (trenches) | 6,000 m and below | None | Snailfish, giant amphipods, microbial mats |
The twilight zone matters more than its gloomy name suggests. Every night, huge numbers of small animals swim up from it to feed near the surface and sink again before dawn. It is one of the largest daily animal migrations on the planet, and it helps carry carbon down into deep water.
How the ocean shapes climate, and what it means for Ireland
Earth’s oceans are the planet’s main heat store. Together they have absorbed more than 90% of the extra heat trapped by greenhouse gases in recent decades, and about a quarter of the carbon dioxide people emit. Currents then move that heat around the planet, which is why coastal climates are milder than inland ones at the same latitude.
The global conveyor belt
Warm surface water flows towards the poles, cools, becomes denser and sinks, then creeps back towards the equator along the seabed. This slow loop through Earth’s oceans, driven by differences in temperature and salt, is called thermohaline circulation. A single parcel of water can take around a thousand years to complete the circuit. In the Atlantic, the part of this system that carries warm water north and cold water south is known as the Atlantic Meridional Overturning Circulation, or AMOC.
Does the Gulf Stream keep Ireland warm?
Partly, but the ocean current is not the main reason Irish winters are mild. That surprises most people, because the popular story says the Gulf Stream is the only thing keeping Ireland from a Labrador-style winter. Research led by the Columbia University climate scientist Richard Seager found that the prevailing winds do most of that work. His American Scientist article on the source of Europe’s mild climate sets out the evidence.
South-westerly winds carry air that has passed over thousands of kilometres of ocean. The sea soaks up heat in summer and releases it slowly through the winter, so that air arrives mild and damp. Eastern Canada, at the same latitude, gets its winter winds from the frozen interior of the continent instead. The Gulf Stream and its extension, the North Atlantic Current, add warmth on top of that, but they are one contributor rather than the whole explanation.
Is the Atlantic circulation slowing down?
Climate scientists agree the AMOC is likely to weaken as the planet warms, but they disagree about how far and how fast. Meltwater from Greenland and heavier rainfall make North Atlantic surface water fresher and lighter, so it sinks less readily. A 2023 study in Nature Communications argued that a collapse could come as early as mid-century. A 2025 study led by Jonathan Baker at the UK Met Office concluded that a full collapse this century is unlikely.
Both camps agree that serious weakening would still bring real effects for Ireland and Britain. The expected changes include cooler, stormier winters, shifts in rainfall and faster sea level rise along the European coast. That is why oceanographers keep instruments moored across the Atlantic to measure the flow continuously.
Where your oxygen comes from
Scientists estimate that roughly half of the oxygen produced on Earth comes from the ocean, mostly from microscopic phytoplankton in the sunlight zone. These drifting algae also form the base of almost every marine food chain. When you watch a green tint spread across the sea off the west coast in late spring, you are looking at a phytoplankton bloom.
The cultural ocean: sea gods, saints and scientists
Long before oceanography, people explained Earth’s oceans through story. Coastal cultures from Ireland to Polynesia gave the ocean gods, otherworlds and voyage tales, and many of those stories are tied to real places you can still visit. They also record something the data tables cannot: what it felt like to live beside water that could feed you or drown you.
Manannán mac Lir and the Irish sea gods
In Irish mythology the sea belongs to Manannán mac Lir (roughly MAN-an-awn mock LEER), whose name means “son of the sea”. He rides across the waves as if they were fields and guards the way to the Otherworld beyond the western horizon. ConnollyCove’s profile of Manannán mac Lir, the Celtic sea god covers his stories and the places linked to him, including the Isle of Man that carries his name.
Irish tradition placed several otherworlds across the western ocean, the best known being Tír na nÓg (roughly teer nuh NOHG), the land of youth. The sea also had its dangers in Irish lore, from the Fomorians to seal-folk who shed their skins on the shore. The seal-folk appear in the legend of the selkies, told on coasts from Kerry to Orkney.
Ireland was never alone in giving the ocean a face. Poseidon, Njord, Sedna and Kanaloa all ruled seas in their own traditions, compared side by side in our guide to ancient sea gods.
Saint Brendan and the voyage tales
Irish monks wrote some of Europe’s earliest ocean adventure stories. The immrama, or voyage tales, follow sailors from island to island across a western sea full of wonders. ConnollyCove’s introduction to the immram tales of Irish literature covers the best-known examples, including the Voyage of Máel Dúin.
The most famous is the Navigatio Sancti Brendani, the voyage of Saint Brendan, written in Latin in the early Middle Ages. It describes a leather-hulled boat, an island that turns out to be a whale, and a promised land far to the west. In 1976 and 1977 the explorer Tim Severin built a leather boat to the medieval pattern and sailed it from Ireland to Newfoundland. His voyage proved the crossing was possible, though not that Brendan made it.
Brendan’s island went on to appear on real sea charts for centuries. It is one of several places mapmakers drew but no ship ever found, a story told in our look at phantom islands that were once believed to exist.
HMS Challenger and the birth of oceanography
Modern ocean science began with a British naval voyage. HMS Challenger sailed from Portsmouth in December 1872 and spent almost four years crossing every ocean except the Arctic. Its crew measured depths with weighted lines, recorded water temperatures and hauled up thousands of species that were new to science.
In 1875, near the Mariana Islands, a Challenger sounding found water deeper than 8,000 metres. The deepest point on Earth, Challenger Deep, takes its name from the ship and from its 20th-century successor. The expedition’s findings filled 50 volumes and set the methods oceanographers used for the next half century.
Exploring the deep: why the seabed is harder to map than Mars
Planetary scientists have mapped the whole surface of Mars in finer detail than we have mapped the floor of Earth’s oceans. Radar and cameras work across empty space, but seawater blocks light and radio waves within a short distance. To see the bottom, you have to bounce sound off it.
Most modern mapping uses multibeam sonar, which sends a fan of sound pulses down from a ship’s hull and times the echoes. Survey vessels cover an area by sailing back and forth in parallel tracks, a pattern hydrographers call “mowing the lawn”. Remotely operated vehicles and autonomous submarines then fill in detail close to the seabed.
The international Seabed 2030 project, a collaboration between The Nippon Foundation and GEBCO, aims to map the entire ocean floor by the end of the decade. The latest Seabed 2030 progress update, published by the IHO, puts the mapped share at 28.7%. That is up from about 6% when the project started in 2017.
Ireland has done more than most countries here. Its national programme, INFOMAR, has been surveying Irish waters for two decades. The territory it covers is many times larger than the island’s land area.
Better maps bring new pressure. Parts of the deep Pacific seabed hold potato-sized nodules rich in nickel, cobalt, copper and manganese, metals used in batteries. Mining companies want to collect them, while marine biologists warn the seabed communities there recover over decades or centuries, if at all. The International Seabed Authority has spent years negotiating rules for commercial extraction.
Protection is growing too. Governments agreed in 2022 to protect 30% of the world’s ocean by 2030. A UN treaty covering biodiversity on the high seas, the waters beyond national control, has also entered into force.
Space still gets more attention than the deep sea, and the two share more history than most people realise. ConnollyCove’s piece on the cultural dimensions of space exploration looks at how people have told stories about the other great frontier.
Where to see the Atlantic from Ireland’s coast
You don’t need a submarine to feel the scale of Earth’s oceans. Ireland’s western edge faces thousands of kilometres of open Atlantic, and on a big swell day the power is obvious from the clifftop.
The Cliffs of Moher in County Clare rise more than 200 metres above the sea and draw the biggest crowds, so arrive early or late in the day. In Donegal, the cliffs in ConnollyCove’s Slieve League guide rise far higher and see a fraction of the visitors. Downpatrick Head in Mayo has a sea stack standing just off the mainland, and Loop Head in Clare has views across the mouth of the Shannon. Slea Head on the Dingle Peninsula looks straight out to the Blasket Islands.
Winter brings the largest swells, when Atlantic storms send waves many metres high onto the west coast. Summer brings calmer seas, longer evenings and the best chance of spotting dolphins, basking sharks and seabirds from the headlands.
Keep well back from cliff edges, which can be undercut and slippery, and never turn your back on the sea on exposed rocks. Freak waves reach far higher than the ones before them. Check the marine forecast from Met Éireann before any coastal walk, and stick to marked paths.
Frequently asked questions about Earth’s oceans
These are the questions people search most often about the ocean, answered briefly with the key figures.
What are the 5 oceans of the world in order of size?
From largest to smallest, Earth’s oceans are the Pacific, Atlantic, Indian, Southern and Arctic. The Pacific covers about 168.7 million km², nearly half of the World Ocean. The Atlantic follows at about 85.1 million km² and the Indian at about 70.6 million km². The Southern Ocean covers about 22 million km², and the Arctic, the smallest, about 15.6 million km².
Is there one ocean or five?
There is one continuous World Ocean, divided by convention into five named basins. Water, heat and marine life move freely between them, and no physical wall separates one from the next. The five names are useful for geography, shipping and science. Oceanographers still treat the whole system as a single connected body of water.
Why is the Southern Ocean considered an ocean?
The Southern Ocean is defined by the Antarctic Circumpolar Current rather than by land. That current flows west to east all the way round Antarctica and keeps the water south of about 60°S colder and less salty than the oceans to the north. The US adopted the name in 1999 and National Geographic added it to its maps in 2021. Its boundaries were proposed to the IHO in 2000 but have not been formally ratified.
How much of Earth is covered by ocean?
The ocean covers about 71% of Earth’s surface and holds around 97% of the planet’s water. Its total volume is roughly 1.3 billion cubic kilometres. Most of the remaining water is locked in ice sheets and glaciers or held underground, with a tiny fraction in lakes, rivers and the air.
What is the deepest part of the ocean?
The deepest known point is Challenger Deep in the Mariana Trench in the western Pacific, at about 10,935 metres. That is deeper than Mount Everest is tall by more than two kilometres. Only a small number of crewed dives have reached the bottom, the first in 1960.
Why is the ocean salty?
The ocean is salty because rivers carry dissolved minerals from rocks into the sea, and evaporation removes the water but leaves the salt behind. Rain is slightly acidic and slowly breaks down rock on land. Vents on the seafloor add more minerals. Sodium and chloride, the ingredients of table salt, make up most of what is dissolved.
Does the Gulf Stream keep Ireland warm?
The Gulf Stream helps, but prevailing south-westerly winds do most of the work. Those winds carry air warmed by the ocean, which stores summer heat and releases it through the winter. Research by the climate scientist Richard Seager found that the atmosphere moves far more heat to Ireland’s latitude than ocean currents do. A weaker Atlantic circulation would still bring cooler, stormier winters.
Who is the Irish god of the sea?
The Irish god of the sea is Manannán mac Lir, whose name means “son of the sea”. In the stories he travels over the waves, controls mists and guards the route to the Otherworld across the western ocean. The Isle of Man is named after him. He appears in several Irish myth cycles, often alongside the Tuatha Dé Danann.
Where can you see the power of the Atlantic in Ireland?
The west coast gives the best views, especially the Cliffs of Moher in Clare, Slieve League in Donegal and Downpatrick Head in Mayo. Slea Head on the Dingle Peninsula and Loop Head in Clare are quieter alternatives. Winter storms bring the biggest waves, but visit with care, keep back from cliff edges and check Met Éireann’s marine forecast first.
Discover more ocean legends
Earth’s oceans are one body of water with five names. The Atlantic on Ireland’s doorstep belongs to the same system as the Pacific trenches and the Arctic ice. Knowing the basic facts changes how you see a coastline. A swell at Slea Head becomes the far end of a journey that may have started thousands of kilometres away.
ConnollyCove’s mythology guides connect ancient stories to the real places where they began. Start with the mythology of the sea and maritime legends, which gathers sea stories from coasts around the world.
For the creatures of Irish folklore, from the Fomorians to the púca, turn to our guide to legendary creatures of Celtic mythology. You can also watch the ConnollyCove YouTube channel for travel films from Ireland’s coast and beyond.
