One Piece Chapter 22, page 14: YOU ARE A RARE AND PRECIOUS ANIMAL TOO

Nami draws a diagram showing how the Grand Line runs perpendicular to the Red Line, wrapping around the entire globe. She explains that it is the most dangerous sea route in the world, and only the Pirate King, Gold Roger, has ever conquered it. Luffy is thrilled, but Gaimon warns them that the Grand Line is known as the "Pirate's Graveyard" and recalls seeing broken, traumatized pirates who returned from it long ago.