Oceans and Seas

Five Oceans and Many Seas

All the oceans are joined, so water and animals can travel between them. Still, geographers split this world ocean into five named oceans: the Pacific, Atlantic, Indian, Southern and Arctic. NOAA, the U.S. ocean agency, recognizes the Southern Ocean as the water south of 60° South, around Antarctica. Not every group agrees on that line, so you may see four oceans in older books.

The Pacific is the giant. It is bigger than all of Earth's land put together. The Arctic is the smallest and shallowest, and much of it is covered by sea ice for part of the year.

A sea is a smaller part of an ocean that is partly closed in by land. The South China Sea covers about 7 million km². The Mediterranean Sea covers about 3 million km² and is over 5,100 meters deep at its deepest point. The Baltic Sea, in northern Europe, averages only about 51 meters deep.

Measuring the very deepest spots is hard, so each survey gives a slightly different number. That is why these depths are rounded.

The Shape of the Seafloor

If you could drain the ocean and walk away from a beach, you would cross a whole hidden landscape. Only about 29 percent of the seafloor had been mapped in high detail with sonar as of April 2026, so scientists are still learning its shape.

1. Continental shelf. A shallow ledge at the edge of a continent, usually less than about 200 meters deep. Sunlight and nutrients make it rich in fish.
2. Continental slope. The steep drop where the shelf ends and the deep ocean begins.
3. Abyssal plain. Wide, flat, dark floor, mostly 3,000 to 6,000 meters deep, covered in fine mud.
4. Mid-ocean ridge. A chain of underwater volcanoes about 65,000 km long, the longest mountain range on Earth. New seafloor forms here as plates pull apart.
5. Trench. A long, deep, narrow valley where one plate sinks under another. Trenches hold the deepest water on Earth.

The average depth of the whole ocean is about 3,682 meters, more than eight times the height of the Empire State Building.

Currents and Gyres

Ocean water is always moving. Surface currents are pushed mainly by steady winds. Because Earth spins, moving water is bent to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This bending is called the Coriolis effect. Together, winds, the Coriolis effect and the shapes of the continents make huge loops of current called gyres.

The Gulf Stream is a fast, warm current on the west side of the North Atlantic Gyre. It flows north from Florida at about 6.4 km/h on average, then turns east as the North Atlantic Current. It carries a lot of heat north. Many books say it alone keeps Western Europe mild. Scientists now think the full story is more mixed. Winds blowing off the warm ocean, and the shape of the jet stream, explain much of why London is milder in winter than places at the same latitude in Canada. Ocean heat adds a few more degrees. Researchers still debate the exact shares.

Scientists also watch a deeper loop called the Atlantic Meridional Overturning Circulation (AMOC). The IPCC says it is very likely to weaken this century, but has medium confidence that it will not collapse suddenly before 2100.

Tides and Waves

Tides are the slow rise and fall of the sea caused mostly by the Moon's pull, with help from the Sun. Most coasts get two high tides and two low tides every 24 hours and 50 minutes. The Bay of Fundy in Canada has some of the highest tides in the world, because its long, narrow shape makes the water pile up.

Most waves are made by wind blowing across the water. The stronger the wind, the longer it blows and the farther it travels over open water, the bigger the waves grow. A wave moves energy, not water: a floating ball mostly bobs up and down as the wave passes.

Who Owns the Sea?

The UN Convention on the Law of the Sea (UNCLOS) sets the main rules. Zones are measured in nautical miles (nm). One nautical mile is 1.852 km. Distances start from the baseline, usually the low-tide line along the coast.

Beyond every country's EEZ lie the high seas, which cover most of the ocean. For a long time there were few rules for protecting life there. A new treaty, often called the High Seas Treaty, came into force on 17 January 2026. It lets countries set up protected areas on the high seas and requires studies before big projects.

Shipping Lanes and Chokepoints

More than 80 percent of world trade by volume travels by ship, according to UNCTAD. Ships follow busy routes called shipping lanes. Many of these routes squeeze through chokepoints, narrow passages where a blockage can delay goods around the world.

When a chokepoint is blocked, ships must take much longer routes. Since late 2023, attacks on ships in and near the Red Sea have sent many ships the long way around Africa's Cape of Good Hope, adding days to each trip. Figures like these change fast, so check the date on any number you see.

Fishing

Fish and other seafood are a key source of protein for billions of people. The UN Food and Agriculture Organization (FAO) tracks the world's fish stocks in a report called The State of World Fisheries and Aquaculture, or SOFIA.

• In 2024, wild-caught fisheries landed about 92 million tonnes, about 80 million of it from the sea.
• Fish farming, called aquaculture, now grows more aquatic animals than wild fishing catches.
• In 2023, about 62 percent of the fish stocks FAO checked were fished at sustainable levels. About 38 percent were overfished, meaning fish were caught faster than they could breed. That share has been slowly rising.
• There is good news too: about 73 percent of the fish actually landed came from sustainable stocks, because many of the biggest fisheries are well managed.

FAO can only assess stocks with enough data, so these numbers are estimates.

Ocean Health

People affect the ocean in several ways. Scientists measure these changes carefully, and the picture includes both problems and progress.

• Warming. The ocean stores about 90 percent of the extra heat trapped by greenhouse gases. NOAA reports that heat stored in the top 700 meters reached a record in 2025.
• Coral bleaching. When water stays too warm, corals push out the tiny algae that feed them and turn white. From 2023 to 2025, about 84 percent of the world's reef area had heat stress strong enough to cause bleaching. Bleached coral is stressed, not always dead, and some reefs held up well and are recovering.
• Plastic. Estimates vary, but studies suggest about 1 to 2 million tonnes of plastic enter the ocean each year. That is about 0.5 percent of all plastic waste. Most comes from places where waste is not collected well, so better trash collection on land makes a big difference.

Countries are acting too. Protected areas, catch limits and the new High Seas Treaty all aim to keep the ocean healthy.

Links to Other Units

• Landforms and Water: rivers, lakes and the water cycle that link land and sea.
• Climate and Biomes: how ocean currents shape climate on land.
• Countries and Borders: how sea borders are drawn when zones overlap.
• Resources and Trade: canal traffic, oil routes and how chokepoint closures affect prices.

Game: Plot the Voyage

Each round names a ship's trip. Tap the chokepoints it passes through, in order, on the usual shortest route. Then press Set sail. Some trips use one chokepoint, some use two or three.

Quiz: Oceans and Seas

Ten questions. Choose one answer for each, then check your score. You can try as many times as you like.

Credits

Written by Marco for Jugami's World Geography course. The maps and diagrams are original SVG illustrations made for this page from the data in the sources. Land outlines come from Natural Earth (public domain). Depths, sizes and traffic figures are rounded, and some are estimates.