The Grand Line is an ocean current approximately located around the entire equator of the world, south of the North and East Blues and north of the South and West Blues. It is separated from the Blues on both the north and south ends by the Calm Belt. The Grand Line is split into two sections by the Red Line: the first section is known as Paradise and the second is known as the New World.
Reputed as the "pirate's graveyard," the sea and the islands possess a vast number of unpredictable phenomena that are not seen anywhere else in the world, many of which make sailing extremely difficult. Additionally, the Grand Line is home to the world's most powerful people and groups, including the Three Great Powers. Only one group, the Roger Pirates, has managed to fully sail across the Grand Line, and this achievement is what caused their captain Gol D. Roger to be labeled as the "Pirate King".
The Grand Line serves as the setting of most of the series.
The Grand Line is the gigantic ocean current that circles around the planet, flowing from west to east. It follows an imaginary line that runs from north-west to south-east across the middle of the world and perpendicular to the Red Line, a vast ring-like continent that circles the globe from north-east to south-west. The Grand Line is placed similarly to the equator. In fact, the main importance of the Grand Line is that it is the only way to circumnavigate the globe.
The Grand Line and the Red Line crossing each other is what divides the rest of the Blue Sea into four oceans: the North Blue, the East Blue, the West Blue, and the South Blue. As such, they are key to worldwide trade and travel in that the Lines are placed along the paths between the Blues. This makes them valuable routes of travel and obstacles for both pirates and civilians alike.
This stretch of ocean is said to be the most dangerous place in the world, and is commonly referred to as the Pirates' Graveyard (海賊の墓場 Kaizoku no Hakaba?) by people from the Four Blues because of this reputation. Despite the reputation of the Grand Line being dangerous and chaotic, many normal people manage a living on individual islands regardless, having adapted to their home land. This indicates that it is actually traveling within the Line itself that is dangerous, as it virtually guarantees facing unpredictable climates and dangerous wildlife. However, there are also several islands, or portions of the islands, which are considered dangerous in their own right, and thus, they are either uninhabited or the citizens have taken precaution to the threat.
The Grand Line is divided by the Red Line into two halves. The first half, stretching from Reverse Mountain to the polar opposite point of the Red Line, is known as "Paradise". The second half, which starts beyond the second Red Line point to the backside of Reverse Mountain, is known as the "New World".
The first half of the Grand Line is known as Paradise ( Paradaisu). It’s name comes from people who have returned from the second half, claiming that the first half is like a paradise in comparison. It lies between East Blue and South Blue.
Paradise stretches from the Twin Cape to the Sabaody Archipelago. Of the few pirate crews that are actually strong and skilled enough to hold their own in the Grand Line, a very small number of them actually make it all the way to Sabaody.
Marine Headquarters is also located near the ending of Paradise
The second half of the Grand Line is known as the New World (新世界 Shinsekai). Its name comes from it being “a whole new world of adventure” compared to the first half. It lies between North Blue and West Blue.
This stretch of the sea is where a number of powerful groups, including the Yonko reside, each holding their own respective territory.
The weather is unusual on the Grand Line to say the least, the standard rules of meteorology don’t apply. The ocean currents and weather patterns are extremely volatile and inconsistent, they can drastically change in an instant; this includes water spouts, storms, blizzards, heat waves, thick fog, and many other such things. There are also cyclones that seem to appear quickly and suddenly, they are remarked as being impossible to predict.
The reason for these erratic currents and weather is because the islands on the Grand Line each have a distinct climate year round, most often falling into one of four basic categories: Summer, Spring, Autumn, and Winter. These types of islands each have four seasons of their own, ranging from Summer Island summer to Winter Island winter, giving the Grand Line a total of 16 different seasons. That’s not including islands with climates that differ from the basic four. It is the clashing weather patterns and magnetic waves generated from each island that causes the strange and unpredictable conditions on the open seas between islands.
Notably, the first leg of the Grand Line is especially chaotic. There are seven different paths that one can choose from to sail through. The clashing weather and magnetism of those first seven islands cause insane sailing conditions that are beyond nearly anything else in the first half of the Grand Line.
The only areas where the weather and currents are stabilized and predictable are the Islands and their nearby surrounding waters. This doesn’t mean the weather will be calm and sunny, it just means that the climate won’t suddenly swift as it does on the open ocean.
A normal compass wont work on the Grand Line due to each island generating its own magnetic field. In order to navigate the Grand Line, one needs a special type of compass known as a Log Pose.
The Log Pose works by locking on to the magnetism produced by one island and points toward that island. After reaching said island and staying there for a certain amount of time, the Log Pose will reset and start locking on to the magnetism of the next island. The time it takes for a Log Pose to set is different on each island; it may take a few hours, it may take a few days, or even a full year.
A Log Pose for the first half of the Grand Line has only a single needle. Alternatively, a Log Pose for the second half has three needles. Each of the three needles locks on to the magnetic waves of one of three different islands. This is because islands in the New World tend to experience changes in their magnetism. Therefore, if you lose the magnetism of one island, you still have two backup options. Furthermore, changes in magnetism can be caused by abnormalities on the island, the unstable magnetism of the island resulting in the corresponding needle jiggling. The trick is to follow the needle that is most stable.
There is another type of Log Pose known as an Eternal Pose. This version of the Log Pose is permanently locked on to the magnetism of a specific island and never resets. This makes it easier for a crew to get to a specific island, rather than following a regular Log Pose through a trail of islands.
As also noted at Water 7, small sea vessels do not make it particularly far in the Grand Line. Despite a crew's best efforts it is difficult to keep a ship in good condition and avoid getting it battered by the Grand Line's natural hazards. Only good quality ships with knowledgeable crews stand a chance of traversing it safely.
Places in the Grand Line
Exact Location Unknown
One cannot sail directly into or out of the Grand Line; to the north and south of the Grand Line are two strips of ocean known as the "Calm Belt". True to its name, the waters and sky of the Calm Belts remain perfectly calm at all times, meaning that there are no waves or currents, there is also no wind or weather, making it near impossible to cross without alternate means propulsion.
In addition to the lack of proper sailing conditions, the Calm Belt is home to a countless number sea monsters, known as Sea Kings; in fact, the Sea Kings found in the Calm Belts are the largest known Sea Kings in the entire world.
The combination of no currents, no wind, and the presence of the Sea Kings makes the Calm Belt a perfect barrier to any traveler trying to directly enter or leave the Grand Line.
Most believe that it is completely impossible to safely enter or leave the Grand Line, save at its beginning and end, Reverse Mountain. However, the World Government regularly sends ships across the Calm Belts by coating their hulls with Seastone, rendering them invisible to the Sea Kings from below.
Without using Seastone-lined ships and alternative propulsion, the Grand Line can only be accessed at two points on the Red Line: Reverse Mountain and the Holy Land of Mary Geoise. The first is where the four major currents of the Blues merge together to form the Grand Line, while the second is the capital city of the World Government.