The flower room is the exact area where One Piece is located, far, far below where it's at under the Red Line.
Reverse Mountain's X is an underwater passageway to a hidden underground system that leads directly to the Flower Room if you follow the right path, but also reveals that the Red Line is hollow enough to travel through, but still solid enough that nothing can break it.
Except for an ancient giant, one with the Deka Deka no Mi.
Blackbeard intends to use Sanjuan Wolf to destroy the Red Line, something Rocks wanted to do with Harald.