Grand Line

The Grand Line is an ocean where the majority of the plot takes place in One Piece. The Grand Line is the ocean current that is surrounded by the Calm Belts and follows an imaginary line that runs from north-west to south-east across the middle of the world and perpendicular to the Red Line. The Red Line is a vast continent that circles the globe from north-east to south-west. These two lines divide the rest of the Blue Sea into the Blues: North Blue, East Blue, West Blue, and South Blue. This stretch of ocean is said to be the most dangerous place in the world, and is commonly referred to as the Pirates' Graveyard by people from the four Blues because of this reputation (But referred to as "Paradise" for those few who make it to the second half, showing it is all relative). Most believe that it is impossible to safely leave the Grand Line save at its beginning and end; however, the World Government regularly sends ships across the Calm Belts by coating their hulls with Seastone, rendering them invisible to the Sea Kings.

The Grand Line is placed similar to the equator. 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.  To the north and south of the Grand Line are strips of ocean known as the "Calm Belts". The true entrance to Grand Line is usually marred by storms; the waters of the Calm Belts remain static year round, and there are virtually no winds or waves (this makes it difficult to cross without propulsion). Extremely large sea monsters called Sea Kings live in the Calm Belts; in fact, the monsters found in the Calm Belts are the largest shown thus far in all of One Piece. The combination of no currents, no wind, and the presence of the Sea Kings makes these belts perfect barriers to any traveler trying to directly go into the Grand Line. Without using Seastone-lined ships (which are concealed from the Sea Kings to a certain degree) and some propulsion, the Grand Line can only be accessed at two points on the Red Line: Reverse Mountain and the Holy Land of Mariejois. The first is where the four major currents of the Blues merge together to form the Grand Line, while the second is the site of the World Government.

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 to those who have been to the New World. It lies between East Blue and South Blue.  While the Grand Line is a dangerous place, the first half is known as such in comparison to the more terrifying dangers present in the New World. This was first mentioned when Luffy remembered Zeff calling the Grand Line a paradise and another time when the pirates wanted to go back to Paradise in the New World. The weather is unusual on the Grand Line. The currents and weather are extremely volatile, as they can change in an instant; this includes water spouts, storms, blizzards, and other such things. Islands and the surrounding waters are the only areas where weather and currents are stabilized in a predictable manner. The reason for this is that islands on the Grand Line fall into four basic categories: Winter, Spring, Summer, and Autumn. These types of islands each have four seasons of their own, giving the Grand Line a total of 16 different seasons. There are also cyclones that seems to appear quickly and suddenly. Although being remarked as unpredictable, Nami was able to sense it coming even in a fevered state.  Normal compasses do not work on the Grand Line because of the nature of its magnetic fields. In order to navigate the Grand Line, a special compass called a 'Log Pose' must be used. The Log Pose works by locking on to one island's magnetic field and then locking on to another island's magnetic field. The Log Pose can do this for a total of seven different routes that diverge from Reverse Mountain and travel east before merging again to the last island, Raftel. The time for a pose to set depends on the island. One island may take from a few hours to a few days to set to the next island, while another can take a year to set. This process can be bypassed by obtaining an 'Eternal Pose'. This version of the Log Pose is permanently set to a specific island and can never change. This makes it easier for a crew to get to a specific island rather than going through a trail of islands. As also noted at Water 7, small sea vessels such as caravels do not make it particularly far in the Grand Line and it was a surprise that the Going Merry had. 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.