I don't know about 50 years ago, but I know when I was in school we were taught that there were 4 oceans. I recently saw an article mentioning 5, and some internet research told me that they had decided on the Southern Ocean as a 5th 20 or so years ago. In fact, IIRC, it was considered an ocean before I was born, was then determined not to be during my schooling, and then made the list again some time thereafter. Make up your mind, people!