How Many Oceans Are There? Four, or Five

All of Earth's saltwater is connected into a single "world ocean", so "how many oceans are there" is a question about how we choose to divide it, not about nature. The answer has shifted from four to five in the last few years.

The five oceans

1. Pacific Ocean — the largest and deepest, covering about a third of the Earth's surface. It is larger than all the land on the planet combined and contains the Mariana Trench, whose Challenger Deep, at about 10,935 m, is the deepest known point in the ocean.
2. Atlantic Ocean — the second largest, an S-shaped basin between the Americas and Europe/Africa. It is still widening by a few centimetres a year as the tectonic plates spread apart along the Mid-Atlantic Ridge.
3. Indian Ocean — the third largest and the warmest of the oceans; bounded by Africa, Asia, Australia and the Southern Ocean.
4. Southern Ocean — the water encircling Antarctica, generally defined as everything south of 60°S. Formally recognised as a distinct ocean by the US Board on Geographic Names and by National Geographic in 2021, though it had been used informally for far longer.
5. Arctic Ocean — the smallest and shallowest, largely covered by sea ice, surrounded by North America, Europe and Asia.

Why the Southern Ocean was added

The Antarctic Circumpolar Current flows clockwise around Antarctica without ever meeting a coastline, and it keeps the water inside it colder, less salty and ecologically distinct from the oceans to the north. Recognising the Southern Ocean as its own body of water reflects that reality. It is not a universal change — many countries, older atlases and school curricula still teach four oceans.

Ocean versus sea

A sea is a smaller body of saltwater, usually at least partly enclosed by land, and is generally considered part of an ocean or connected to one. The Mediterranean, the Caribbean, the South China Sea, the Arabian Sea, the Baltic and the Black Sea are all seas. The Caspian Sea is the odd one out — despite the name, it is landlocked and is technically the largest lake in the world. The Dead Sea and the Sea of Galilee are also lakes.

A few ocean facts

• The oceans cover about 71% of the Earth's surface and hold about 97% of its water.
• The Pacific is so large it once had all the continents on one side of the planet (as the supercontinent Pangaea) and open ocean on the other.
• The average depth of the ocean is about 3,700 m; if the Earth were perfectly smooth, water would cover it to a depth of about 2,700 m.
• More than 80% of the ocean has never been mapped in detail or directly explored.