Thanks!
The entire map can be filled! The method to accessing it is a tad bit unfair to the player. If I had made the game now, I wouldn't have done it like this. There is a hint or two on how to access it in the frog village. It is something that can be missed on a first playthrough, so you may have to start a new save to access the missed content. I will also add that the actual reward you get isn't exactly worth the effort... there's videos online if you'd like to see someone complete it without doing it yourself. I wouldn't blame you for skipping it.