Reverse Mountain, One Piece Wiki

Reverse Mountain is an extremely tall mountain at one of the points where the four Blues meet and is one of the two known entrances to the Grand Line. It is located in the Red Line and is at the opposite side of the world from Mary Geoise. The Reverse Mountain Arc takes place here, specifically in the Twin Cape, the bottom of Reverse Mountain on the Grand Line's side. This place is unique in that rivers from the Blues flow up the mountain, meet at the top, and then flow down into the Grand Line.

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