One Piece Chapter 51, page 3: Roronoa Zoro Falls Into the Sea

Zoro clenches the sword in his mouth, glaring at his opponent, and asks, "Just what're you planning to do with that?" Mihawk has pulled a tiny, cross-shaped dagger from his necklace with a sharp "SHING" sound. Mihawk explains, "I'm different from those stupid brutes who'd go all out just to hunt a mere rabbit. Though you may be a swordsman of some renown, this is East Blue, the weakest of the four seas..." A map of the world appears, showing the four seas—North Blue, West Blue, East Blue, and South Blue—separated by the Red Line and the Grand Line.