sunken cave

[crazybayman]

It is the Cavern of the Stolen Fayth. Which is near the entrance of Mount Gagazet.

Go to the Calm Lands and head out through the northeastern exit, where you left when
you came through here on the way to Zanarkand. Go across the first bridge,
then go around and down the hill under the second. If you recall, when you
tried to enter here after you fought Defender X, your other party members
told you it was the wrong way and you were not allowed to enter. It only
opens after you obtain the airship.

Is that in the international version? Because (in my NA version) I've always went on to the Cavern of the Stolen Faythe immediately after defeating Defender X. Yeah, my companions said it was the wrong way, or dangerous, or something of that nature, but I was still able to go down anyways.

But yeah, that's where the Sunken Cave is. If you do it after obtaining the airship, it should be a joke.