Final-fantasy-vi-codex -

Mog can only learn the "Water Rondo" dance by fighting a battle in the Serpent Trench or Lete River during the World of Balance. If you do not get it before the world ends, you can never complete his dance list.

In the early caves (like the Cave to South Figaro), leaving chests unopened during the World of Balance causes them to upgrade into incredibly powerful endgame gear when opened in the World of Ruin. 🔮 The Esper & Stat Growth Codex final-fantasy-vi-codex

A dedicated "Final Fantasy VI Codex" (like a built-in encyclopedia or bestiary) does not exist in standard editions of the game; instead, Mog can only learn the "Water Rondo" dance