After uncovering the location of Pluton from the Poneglyph of Alabasta, I swam over to Fishman Island in search for the next Poneglyph.
The ocean’s floor bed was rather dark and difficult to trek through. I chanced upon a giant hermit with a shell made of craggy rock formation and hitched a rather uncomfortable ride through the underwater terrain.
I finally reached my destination. The Fishman Island Poneglyph revealed that Mermaid Princess Poseidom was one of the 3 ancient weapons.
(One Piece Reference)