I thought just struck me. Check it out:
There is a stream coming from each of the Four Seas. At the top they collide and the water flows down into the Grand Line to the Twin Cape. So far everything's fine. But... There is no stream coming from the other side of the Grand Line, where Raftel is supposed to be. So unless there is a way UNDER the mountain...
How the hell are the Straw Hat pirates supposed to get back to the Twin Cape once they are on Raftel?
I doubt it's just because the map is incomplete at that point.
"Okay, guys! We found One Piece but it looks like we have to sail all the way back." "You're kidding, right?" "But Laboon-" "Shut up, Brook, and get on your post!"