One ocean with five names
The world's oceans are really one connected body of salt water covering about seventy per cent of the planet, divided into five named oceans for convenience. The PACIFIC is the largest by far, bigger than all the land on Earth put together, and the ATLANTIC is second. The INDIAN is the warmest, and the ARCTIC the smallest and shallowest, much of it covered in floating ice. The SOUTHERN Ocean around Antarctica was only officially recognised as the fifth ocean by the National Geographic Society in 2021, although scientists had used the name for years.
The Atlantic is growing: North America and Europe are drifting apart by a couple of centimetres a year as new sea floor forms along a mountain chain down the middle of the ocean.
Seas, straits and a sea that is a lake
A sea is usually a smaller part of an ocean, partly enclosed by land, such as the CARIBBEAN, the ARABIAN Sea, the BALTIC or the MEDITERRANEAN. A STRAIT is a narrow channel between two areas of land; the BERING Strait, between Russia and Alaska, is only about 80 kilometres wide, and during the last Ice Age, when sea levels were lower, it was dry land that people and animals walked across. A GULF is a large bay, like the Gulf of Mexico.
The CASPIAN Sea, between Europe and Asia, is not really a sea at all but the world's largest lake, with slightly salty water and no link to the ocean. A LAGOON is shallow water cut off from the sea by a reef or a bank of sand, and a HARBOUR is a sheltered spot where ships can shelter along the SHORE.
Why the sea is salty
Rain is fresh water, but as it runs over the land in rivers it dissolves tiny amounts of minerals out of the rocks and carries them down to the sea. When sea water evaporates, it leaves the salt behind, so over hundreds of millions of years the oceans have become salty. On average about three and a half per cent of sea water is salt. If all the salt in the oceans could be taken out and spread evenly over the land, it would make a layer more than 150 metres thick, taller than a forty-storey building.
The least explored place on Earth
The deepest point in the oceans, the Challenger Deep in the Mariana TRENCH in the western Pacific, is nearly eleven kilometres down, deep enough to swallow Mount Everest with more than two kilometres to spare. Only a few people have ever been there; our submarines page tells the story of the first dive. Most of the SEABED has still not been mapped in fine detail, and scientists often say we have better maps of the surface of Mars than of our own ocean floor.
Water that never stops
Water in the oceans is always moving. Winds drive surface CURRENT systems, and a slow, deep circulation carries cold, salty water from the poles along the sea floor, taking around a thousand years to go round the globe. Every WAVE, every TIDE and each coral REEF depends on that restless water. The MARINE life that lives in it, from the CORAL Sea to the open ocean, is on our sea animals page.