Historical Interpretation
"The Seven Seas" is a very old term, going back to 2300 BC. Back then, many of the world's oceans hadn't been "discovered", so "seven seas" sort of meant "All the seas of the world".
The term "Seven Seas" appears as early as 2300 BC in Hymn 8 of the Sumerian Enheduanna to the goddess Inanna. The Mesopotamians were the first in the history of astronomy to keep records of the observed seven moving objects in the heavens – the seven Classical Planets/Seven Heavens – and they made this connection to their seven seas.
https://en.wikipedia.org/wiki/Seven_Seas
The number seven has always been mystical and magical, and thus the theory is that it was just chosen out of the air, and later people(s) applied the names of actual "seas" (or other waterways) to come up with seven. Different cultures from different ages and civilizations have different sets of seven "seas".
Modern Interpretation
Since the 19th century, the term has been taken to include seven oceanic bodies of water:
1. the Arctic Ocean
2. the North Atlantic Ocean
3. the South Atlantic Ocean
4. the Indian Ocean
5. the North Pacific Ocean
6. the South Pacific Ocean
7. the Southern (or Antarctic) Ocean
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