u/Aetherflaer

Try to picture the inside of Reverse Mountain as a piece of Ancient Tech that used to be able to be rotated in order to control the currents of the oceans. The Redline was created to stop Reverse Mountain from Rotating and lock it in place so that the ocean currents are permanent.

tl;dr: The One Piece, is just "one piece"of the boardgame that went missing. It determines the playing field for the 3rd game of the board game of the Davy Back Fight between Imu and Davy Jones. That's why Roger laughed.

Please see my previous post about the Void Century here if you care. It might help a tiny bit.
https://www.reddit.com/r/OnePiece/comments/1t3u7hw/the_actual_secret_of_the_void_century/

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If you read the final game of Long Ring Long Land as an analogy for the set up for the Third World in the mural at Elbaf things start to make perfect sense.

Each time the world is remade it is setting the stage for a level of a Davy Back Fight between Davy and Imu. Just like they change stages for each fight in the Davy Back Fight in LRLL.

World 1 in the mural was the first Davy Back Fight. Imu lost this one.
World 2 is the Second Davy Back Fight. Imu won this one. But chose Davy Jones as the person to join his crew. This made Davy join Imu and canceled the third game.
World 3 is the third Davy Back Fight, Imu Flipped the table and the board game pieces went flying.

Long Ring Long Land Haters in Shambles.

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