The "Southern Ocean" is not one of the Big 3 but it is important in an oceanographic sense.

Well, color me naive, I've never seen a map with the Southern Ocean labelled. Someone's hiding things from us.

Seems like it would be just as big, or bigger than the Arctic Ocean, so why wouldn't it be a major one? Are there any other "oceans" that have been hiding from me? And also, are the noticable boundaries between the oceans, so that you can actually tell in a boat, that you've gone from one to another?