We’ve all heard the old adage of how the pirates of yesteryear sailed the “seven seas”, but exactly how many seas, or oceans, are there? Well it depends on who you ask, so let’s start at the smallest number and work up.

One ocean

Most people agree that there is one ocean composed of salt water, covering about 71% of the Earth’s surface and accounting for 97% of all water on the planet. This water is connected by the “global ocean conveyor belt”: a system of currents that move water, minerals, organisms and heat around the world. These currents are driven by several factors including the moon, the Earth’s rotation and the water’s salinity, temperature and density.

However, it can sometimes be convenient to divide the whole ocean up into smaller sections, just like how we divide land up into countries or continents. There are a few ways to do this.

Three oceans

The first way is by looking at how the Ocean is split into three by the large land masses of the Americas, Africa and Oceania. Using this system, the named oceans are the Atlantic Ocean, the Indian Ocean and the Pacific Ocean.

Four oceans

At very high latitudes, the ocean tends to be covered by ice, for at least part of the year, which is why the Arctic Ocean is considered by many people to be the fourth ocean.

Five oceans

In the year 2000, the International Hydrographic Organisation proposed that the water surrounding Antarctica be added to the list as the fifth ocean called the Southern Ocean. However, just like on land, some countries did not agree with where the boundaries of this ocean should be. Many countries still choose to recognise the Southern Ocean even though the proposal hasn’t been officially ratified.

'Seven seas'

The so called ‘seven seas’ have changed over time. Since the 19th century, their distinction is a result of separating both of the Atlantic and Pacific Oceans into North and South subdivisions. Thus the seven seas would be the North Atlantic, South Atlantic, North Pacific, South Pacific, Arctic, Indian and Southern Oceans.

Other seas

What about seas like the Mediterranean Sea or the Red Sea? They are actually just parts of the larger oceans. The Mediterranean Sea is part of the Atlantic Ocean as it is connected through the Straits of Gibraltar and the Red Sea is part of the Indian Ocean.

Inland seas

There are also some famous so called ‘inland seas’, such as the Caspian Sea and the Dead Sea. These are actually considered to be lakes because they aren’t connected to the rest of the ocean.