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 the geographical antipode of Mary Geoise.
The mountain has five rivers, four ascending from each of the world's seas and one descending into the Grand Line.
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. This is because the mountain, being a "Winter Island" using Grand Line terminology, is much colder than the surrounding sea in the Blues but not in the Grand Line. Many pirates entering the Grand Line die before they can even get up to the top of the mountain, because the hostile water currents and storms force them to crash into the mountain walls.
On the entrance of the river are ten square arcs that are decorated with tribal patterns and depictions of sea creatures. The Reverse Mountain Arc takes place here, specifically in the Twin Cape, the bottom of Reverse Mountain on the Grand Line's side.
- Similar tribal patterns with those found in the Reverse Mountain gates are a common sight in the ruins of Shandora.
- In the 15th One Piece intro "We Go!" a crossbone symbol common in Jolly Rogers is displayed, which then transforms into the map of the reverse mountain. However, the connection between these two remains unknown.