You meant Crocodile and Robin? Well it happen after Crocodile got his asked kicked if not there it will be in the flashback when Robin joined the crew. The conversation was between Robin and Vivi's father. Not Crocodile.
Originally Posted by arisart
She says while reading the Skypiea poneglyph. "Poseidon....and it speaks of its location...who would had thought this dangerous thing still out here."
When you find the Alabasta one it will explain the ponyglyphs by Robin to the vivi's father. Like guy above said there are only 2 types. Ones with information and ones with locations.
Edit: Thanks for finding out about the poneglyph in the flashback, now I know Robin at least read 3. Just something that I'm adding up for later use.