One Piece Grand Line
The Grand Line is the ocean current that follows an imaginary line that runs east to west across the middle of the world and perpendicular to the Red Line. In effect, it's the equator in the world of One Piece. The Red Line is a vast continent that circles the globe from north to south and functions as a Prime Meridian. These two Lines divide the two oceans of the world into four seas or 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 sometimes referred to as the Pirates' Graveyard because of this reputation. There also appears to be ways of how the Grand Line can be left safely without sailing to the far end of the Grand Line. How that works hasn't been explained as of yet.
Calm Belt
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 either. There is also the presence of extremely large sea monsters (called Sea Kings) ; in fact, the monsters found in the Calm Belts are the largest, or at least largest shown thus far, in all of One Piece. The combination of no currents, no wind, and many Sea Kings make it a perfect barrier for any traveler trying to directly go into the Grand Line.
Without using seastone-lined ships (which repel the Sea Kings) and some propulsion, the Grand Line can only be accessed at two points on the Red Line: Reverse Mountain and the Holy Land of Mariejoe. The first is where the four major currents of the Blue Seas merge together to form the Grand Line while the second is the site of the World Government.
The New World
The Grand Line is divided by Red Line into two halves. While the first half (stretching from Reverse Mountain to the polar opposite point of the Red Line) is yet unnamed, the Second Half (beyond the second Red Line point to the backside of Reverse Mountain) is known as the "New World". This is the part of the world where the Yonkou live and rule like Emperors, and where those who will begin the new era gather. This half has not been fully explored, save for by the legendary Pirate King Gol D. Roger himself.
Navigation
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 in 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 a few hours to set to the next island, while another can take a year to set. This 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.
