This article is from the Final-Fantasy and other Square Soft Games FAQ, by firstname.lastname@example.org (Nick Zitzmann) with numerous contributions by others.
Head back to Figaro Castle later in the game once you have the Airship
and a considerable amount of experience, and submerge the castle. On the
way back to the Figaro Desert, the engineers will report that the castle
has run into something. Stop and explore.