Episode 555 - The Great Showdown! Luffy and His Crew vs. the Strongest Pirates

Episode 555
The Great Showdown! Luffy and His Crew vs. the Strongest Pirates
Japanese: 大決戦！ルフィと仲間たちVS最強海賊
Dai Kessen! Rufi to Nakama-tachi VS Saikyō Kaizoku

The crew successfully defends the artifact, but in doing so, they uncover an underground network that connects multiple islands across the Grand Line. The crew must now navigate this network to find the treasure before other dangerous pirates can use the network for their own gain.