The fictional world of One Piece is covered by two vast oceans, which are divided by a massive mountain range called Red Line. The Grand Line, a sea that runs perpendicular to the Red Line, further divides them into four seas: North Blue, East Blue, West Blue and South Blue. Surrounding the Grand line are two regions called Calm Belts, which experience almost no wind and ocean currents and are breeding ground for the huge sea creatures called Sea Kings, because of which the Calm Belts are very effective barriers for those trying to enter the Grand Line. While the navy ships, using sea stone to mask their presence, can simply pass through, most have to use the canal system of Reverse Mountain, a mountain at the first intersection of the Grand and Red Lines. Sea water from each of the four seas runs up that mountain and merges at the top to flow down a fifth canal and into the first half of the Grand Line. The second half of the Grand Line, beyond the second intersection with the Red Line, is also known as the New World.
The currents and weather on the Grand Line's open sea are extremely unpredictable, while as in the vicinity of islands the climate is stable. What makes it even harder to navigate is the fact that normal compasses do not work there. 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 time for it to set depends on the island. This process can be bypassed by obtaining an Eternal Pose, a Log Pose variation that is permanently set to a specific island and never changes.
Devil Fruits are a type of fruit which when eaten confer a power on the eater. There are three categories of Devil Fruit. Zoan fruits allow the user to fully and partially transform into a specific animal. Logia fruits give control over and allow the user "to change their living body structure into the powers of nature". Paramecia is a catch-all category for fruits that give the user superhuman abilities. Devil Fruit users cannot swim. When even partially submerged in water, they lose all of their strength and powers.