Reverse Mountain is an important geographical feature in the world of One Piece because it represents the central entrance to the Grand Line, which originates from vessels in the four blues: East Blue, West Blue, North Blue, and South Blue. This gigantic mountain, with an enormous height of 10,000 meters or 10 kilometers, serves as a portion of the Red Line, the huge continent circumnavigating the globe.
The most unique and apparently impossible feature of Reverse Mountain is its upstream rivers. These rivers flow against the force of gravity, brimming with water and ships from surrounding seas up the mountain and into the Grand Line. This has often been described by characters in the series as "magical"; Monkey D. Luffy frequently expresses a sense of awe at the mountain's nature during the Straw Hat Pirates' ascent.
The Reverse Mountain Arc presents a major turn in the storyline because it represents the entry of the Straw Hat Pirates into the Grand Line. Not to forget other reasons, this arc adds important elements vital for the progression of the series, such as the Log Pose, a type of compass especially needed in order to steer through the Grand Line. Knowing the series of events, its placement in the story thus underscores the reputation of the Grand Line as a treacherous and mysterious sea, which will be the backdrop for their future adventures.
According to the anime adaptation, this Reverse Mountain Arc was preceded by a filler storyline involving Warship Island not found in the original manga. Therefore, the addition is thus in the anime for foreshadowing purposes of entering into the Grand Line and allowing more character development before entering into the coming challenges of Reverse Mountain.
One Piece fans appear to love to debate about the physics of Reverse Mountain. The series does explain the upward flow of water due to the lack of currents and winds in the Calm Belt, hence raising the sea level in the four Blues compared to the Grand Line. Added to this difference in sea level are the strong currents formed around the Red Line, which creates the upward flow. Though this explanation challenges real-world physics, this only enhances the fantastic nature of the One Piece world.
Reverse Mountain also plays a crucial role in the world's geography and politics. Its position at the entrance to the Grand Line makes it a strategic point, controlling access to and from this important sea route. The mountain's unique properties make it one of the few safe ways to enter the Grand Line, as the alternative would involve crossing the dangerous Calm Belt.
Perhaps one of the most memorable aspects of the introduction of this arc is Laboon, the great whale who awaits below the mountain, waiting for a pirate crew that he has not seen for decades. Encounters like this present not only emotional subplots in the story but also foreshadow future events and characters of the series, proving Eiichiro Oda's mastery at long-term storytelling.
The visual depiction of Reverse Mountain in the manga and anime is amazing, often visualizing the mountain as an impossibly tall structure with rivers climbing upwards along its sides. Dramatic climb of the Straw Hat Pirates upwards the mountain visually stands out as a very beautiful moment in the series to impress on the reader or viewer the monumentality of what the Straw Hats are about to do.
This position of Reverse Mountain in the One Piece world is what has led to many fan theories respecting its importance in the broader narrative. Some believe that the proximity of it to both the beginning and potential end of the Grand Line might relate to the location of Raftel, the last island on which the treasure of One Piece is said to be hidden. Even if these theories are not confirmed, it shows how deep the worlds that Reverse Mountain has added into the series.