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.
Not much is known about Reverse Mountain, other than the fact that many pirates try and use it to enter the Grand Line on a daily basis.