Reverse Mountain is a mountain at one of the points where the four Blues meet and is one of the two known entrances to 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 due to the fact that 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.[1]
On the rivers' borders are several gates built by an unknown hand.
Twin Cape
Twin Cape is located at the bottom of Reverse Mountain, at the exit that leads into the Grand Line. The area is located precisely where the Red Line and the Grand Line meet. From this place is where one can choose one of the seven routes of the Grand Line by waiting for the Log Pose to set on the desired one.
The lighthouse is located at the bottom of reverse mountain, where one can find information about the dangers of the Grand Line. There's a lighthouse on both sides of the entrance, and Crocus appears to reside at the northern one.