Quick answer
When was the first Poneglyph shown?
Verified through chapter 202Updated
What the manga shows
5 factsRobin and Crocodile descend into the underground chamber where the Poneglyph is housed.
Robin stands before the massive inscribed block in the circular stone chamber.
Robin and Crocodile descend a long dark stone staircase into the royal crypt.
The poneglyph is housed deep down in an underground chamber.
Inside the vast circular stone chamber Robin stands before the massive inscribed block.
Change log
1 chapter- Chapter 202Added 5 facts; updated the answer