The Grand line is a stretch of ocean to the far south of all the continents, and is thought to be the edge of the world. It is a wild and unpredictable place, with a dense dark fog covering it at all times, and freak storms of all types are known to burst out without warning. While the Grand Line itself is cold, the islands which happen to lay within close proximity to it, have numerous different climates, so diverse that no two islands of the same type or conditions can be found within a thousand miles of another island of the same climate. Also near the Grand Line, are several perpetual storms known by such names as the crackling Eye of Thead, the frozen Hand of Beation, and Da Fut o' da Groove o' Tings (best not to esquire as to what its nature is). These storms are so constant, that their positions can actually be used for navigating ships, and are often better navigation aids then stars or the sun.
While large numbers of people believe it is the edge of the world, there is a growing school of thought (mainly in Garmaria), that the Grand Line is actually just some sort of ring on the world to divide it spheres. Such a idea does have credit, as on oddly calm days, sailors have reported seeing ocean on the other side of the Grand Line, and Dwarven Tradition maintains that the the world is square and they have sent numerous aero-clads to prove just that. However, time and time again, the age old wisdom that it is best to leave the Grand Line alone has proven itself an infinite number of times. Recently some fishermen did report seeing black ships leaving the Grand Line, creating even more interest in the Grand Line then ever before. Perhaps one day some lucky ship will actually make it through the Grand Line and survive to tell what happened.