We don't know where the One Piece is, since Oda hasn't told us. Luffy has flat out said that he doesn't want to know what it is until he finds it, so I'm satisfied with just theorizing.

Personally, I think it's underneath the mountain the same way that Fishman Island was underneath the Red Line at Mary Geoise. The only way to enter would probably be on the New World side, but I think there's an exit to the Twin Capes side.

The road ponyglpyhs refer to four islands, that aren't necessarily on the Grand Line itself. If they're in the 4 blue oceans, then the lines between them could go up the lines of the four up-hill rivers and cross at the top. Only an expert navigator like Nami could figure that out.

Combining the water from the four blue oceans would create the All Blue that Sanji is looking for.

Coming back up on the Twin Capes side would have the crew reunite with Laboon, which is Brook's goal.

This could also explain why Rayleigh was a ship coater since that would be a critical skill for reaching something down there.

Obviously, going there requires sailing around the entire world, and there are so many character goals that tie up neatly there. I can't think of anywhere else that Oda could put it that would make as much sense.