After defeating Gabriel Celeste in the Cave of trials, he'll tell you that "the seal has been broken". What this means is that you can now head to the Sphere Corporation for the secret dungeon. (However, you might want to head to Urza Cave Temple first; there are some items there that may help, and it's a nice transition dungeon between the Cave of Trials and Sphere 211).

Sphere 211 is so Big, so this 'placeholder' with be in effect for a while