It might be described that way because all of the oceans on this planet are connected, therefore form one huge ocean.
This is probably because all the oceans help us by giving rain and all the oceans are connected with each other. Hope this helps you.
Earth's oceans are described as one huge ocean because they are interconnected through currents, share similar chemical compositions, and have historical unity as a single body of water. This vast connection influences global climate and ecological systems. Thus, their unified nature is critical for understanding Earth’s environment.
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