Reverse Mountain Arc

Sometimes called the Laboon arc, this is the seventh story segment and the first of the Arabasta Saga. At five chapters it stands as one of the manga's briefest entries. The crew finally tackles the only sanctioned route into the Grand Line, a mountain whose rivers flow upward.

The chapter pairs spectacle with melancholy, introducing the lighthouse keeper Crocus, the heartbroken whale Laboon, and the mysterious agents Mr. 9 and Miss Wednesday, whose identities and organization will shadow the crew for the rest of the saga.

Caught in a storm, Nami realizes their entry point demands a climb up Reverse Mountain. The ship drifts into the windless Calm Belt, where Sea Kings nearly sink them, before they scramble back and ride the rushing canal upward. Luffy braces the Going Merry to keep it from smashing apart, and the crew plunges down into the Grand Line at last.

They immediately collide with Laboon, an enormous whale blocking the route who swallows the ship whole. Inside, the crew meets Crocus, who tends to the creature. The whale has been ramming his head bloody against the mountain for over fifty years, still waiting for a pirate crew that left him behind. Luffy, after a fierce exchange of blows, declares himself the whale's rival and paints his Jolly Roger on Laboon's brow, promising a rematch once he circles the Grand Line.

Crocus explains the Log Pose and how the Grand Line's islands each carry unique magnetic fields leading toward the final isle, Laugh Tale. After fending off the assassins Mr. 13 and Miss Friday, the two agents persuade the crew to ferry them home to Whisky Peak. Bidding Crocus and Laboon farewell, the Straw Hats sail onward as Crocus wonders whether Luffy is the one he and Roger long awaited.

The crew has officially entered the Grand Line and gained a Log Pose to navigate it. Luffy's vow to reunite with Laboon plants a seed that blooms much later when the Straw Hats meet Brook, the last surviving member of the whale's old crew, who joins them to keep that promise. It is also revealed down the line that Crocus once sailed with the Roger Pirates and spent years searching for Laboon's lost companions.

By agreeing to escort the two agents to Whisky Peak, the crew unknowingly draws closer to Baroque Works, the organization that will drive much of the Arabasta Saga. Key Grand Line lore, including the Calm Belt, the Log Pose, and the location of One Piece on Laugh Tale, is established here for the first time.