The Red Line is a massive ring-shaped landmass that circles the entire One Piece planet and acts as a wall, dividing the globe. Combined with the Grand Line, it divides the sea into four quadrants, namely East Blue, North Blue, West Blue, and South Blue.
The Red Line is constructed of a reddish mineral and it is claimed to stretch 10,000 meters underneath the sea's surface to Fish-Man Island on the ocean floor. At the same time, the Red Line's summit is sufficiently above sea level to be regarded as insurmountable. The Red Line is unbreakable as well. In addition, the Red Line, which spans the Grand Line, has its own "islands" for the seasons of summer, winter, spring, and fall, separating its weather conditions from the rest of the world.
Although it is unpassable, restricting passage to the Grand Line, there are some passageways and methods to cross the Red Line. Reverse Mountain is referred to as the entrance of the Grand Line among pirates wishing to enter it. Although it is not available for pirates, one could also hope to cross the Grand Line by gaining access through the World Government and passing through the Holy Land of Marijoa.
Even though it is impassable, there have been a few individuals who were able to climb it. Fisher Tiger managed to climb the Red Line barehanded to attack Marijioa to free the slaves from the grasp of Celestial Dragons. Additionally, Germa 66 also managed to climb the Red Line with their amphibious ships to travel from North Blue to East Blue.
It is stated that long, long ago, before Mary Geoise was built, there was a race of people that resided above the Red Line, known as Lunarians. At that point in time, it was known as the Land of Gods. Lunarians had a set of unique attributes that set them apart from the rest of the people in the One Piece World. It is stated that Lunarians had black feathered wings, dark skin, white hair, and the innate capacity to produce flames from their bodies. Lunarians are commonly thought to be extinct, with King (a member of the Beast Pirates) being the only known living member.
Volume 3 Chapter 22: You’re the Rare Breed (Mentioned)
Volume 12 Chapter 101: Reverse Mountain (Visualized)
Episode 3: Morgan vs. Luffy! Who's This Mysterious Beautiful Young Girl? (Mentioned)
Episode 63: A Man's Promise! Luffy and the Whale Vow to Meet Again (Visualized)
Mary Geoise, the World Government's capital city, is located at the intersection of the Grand Line and the Red Line. It is said to be located in the center of the One Piece world.
The Red Port has customized elevators called Bondolas that go to Mary Geoise and let people cross the Red Line through Mary Geoise if they have access to do so. It can be found at either end of the Grand Line, at the base of the Red Line, and directly beneath Mary Geoise.
Reverse Mountain, which lies on the other side of the world from Mary Geoise, is where water from the four Blues, namely North Blue, East Blue, West Blue, and South Blue flows up the mountain and into the first part of the Grand Line, known as Paradise.
A. The New World is the name given to the second half of the Grand Line. It is located between West and North Blue, as well as Fish-Man Island and the other side of Reverse Mountain.
A. The Grand Line is an ocean current that roughly circles the entire planet's equator. The Calm Belt separates it from the Blues on both the north and south ends.