Instructions
Surf south from Route 119 until you reach a large sandbar on the border of Routes 119 and 120.
Stand on the sandbar and walk to its southern edge. Pick up the Splash Plate.
Use the Explorer Kit item to enter The Underground area. You can use the Explorer Kit to tunnel into The Underground from any outdoor surface area while standing on dry land. Dig into the bulges in the walls of the expansive maze to randomly find Splash Plates. Splash Plates are a very rare item to find in The Underground, so be patient and dig often.