One Piece Grand Line: Where Is The Deadly Ocean?

We've heard mention of the Grand Line in Netflix's One Piece, but where is this line of ocean and why is it so important?

My take on One Piece's world size. Slide for the calculations and

ONE PIECE TREASURE CRUISE - Apps on Google Play

The World Map of One Piece

Top 10 Worst Islands on the Grand Line

One Piece: What Exactly Is The 'Grand Line' And Why Is It So

Grand Line One Piece by Giamini on DeviantArt

One Piece: The powers and abilities of the Sea Kings explained

I just learned it is possible to travel through the calm belts

World, One Piece Wiki

My take on One Piece's world size. Slide for the calculations and