The Grand Line (偉大なる航路 [グランドライン], Idai Naru Kōro [Gurando Rain], literally translated as Great Sea Route) is an ocean where much of the story 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 its 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 Kairoseki, rendering them invisible to the Sea Kings. The Grand Line is placed similar to the equator.
