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?

Why cant the marines/powerful pirates just cross through the calm

Grand Line, One Piece Wiki

Discussion - How do people leave grand line.

How to Watch One Piece in Order (Including Movies) - IGN

The Treasure Hunt

One Piece: The Klabautermann of Going Merry expained - Dexerto

One piece planet size : r/OnePiece

In the One Piece universe, what is stopping anyone from flying

The Best One Piece Story Arcs, Ranked