The Seven Seas

"Sail the seven seas" goes back to very ancient times, perhaps as long ago as 2100 BC or earlier. Even then, the term described an adventure of huge proportions, navigating a vessel on all of the major known waters of the day. There is no absolutely correct list of those seven seas, but they must have been the ones surrounding the cradle of civilization:

the Red Sea
the Mediterranean Sea
the Persian Gulf
the Black Sea
the Adriatic Sea
the Caspian Sea
the Indian Ocean

Some geographers suggest that the Aegean Sea and the Arab Sea should be considered part of this list.

In more recent times intrepid sailors ventured out on even greater bodies of water, so huge that there seemed to be no end to them. They became known as the four oceans:

the Atlantic Ocean
the Pacific Ocean
the Indian Ocean
the Arctic Ocean

Notice that the Indian Ocean appears on both lists. There are also other lists of named areas of the earth's waters, but they actually represent subdivisions of these major oceans.

For example, some insist that there must be a North Atlantic and South Atlantic Ocean, as well as a North Pacific and South Pacific Ocean, and that makes a lot of sense too. Some others reference the Southern Ocean, which describes the waters south of the continents.

There are many other large bodies of water known as seas, such as the Dead Sea, as well as some very sizeable lakes and bays, but they don't usually fit into this rather poetic terminology: the four oceans and the seven seas.