User blog comment:Balbal1/End of the Grand line/@comment-24769684-20160322184906

There isn't a Road Poneglyph on there, otherwise someone would know where it was (unless no one's ever managed to reach the end of the route, which I doubt).

I can't really think of anything off the top of my head of what the last charted island could be, but I have two theories. One is that it's the wildest island in the world in terms of climate, as the fact that EVERY log pose goes to it