You needed ten Espers, and then you had to speak to an NPC in the village on the far side of Ozmone Plain (he wasn't hard to find, and implied the quest if you spoke to him beforehand). That opened up Henne Mines Site B; you had to go down to the Ore Separation Facility where you fought the dragon, and turn right instead of left (which took you back to the surface). You then had to crawl through a long and difficult dungeon filled with dozens of powerful monsters that, in any other area, would be mini-bosses. A few switch-based puzzles and a labyrinth mine traversal later, and you found a lone save crystal, and Zodiark on the other side. And the fight made everything you just went through seem like replaying the first area of the game. Assuming you survived (Zodiark's attacks had a chance to cause instant death, regardless of stats), the game then deposited you in the middle of the Feywood, and while you would normally coast through the area, your characters would be seriously weakened.