Actually, when you first retrieve the Earth Crystal, the party was Cecil, Yang, Tellah, and Cid... The crystal you're talking about was one of the underground Dark crystals... the Sealed Cave to be exact. Thats the dungeon with all the trap doors.... and I'm pretty sure it's Rydia, Warp, After beating Golbez and going to the throneroom of the dwarven castle... heard about it and I did it once out of sheer luck...
*edit: and actually, I had a savefile right after I crashed for the first time underground *