I'm pretty sure this is the Unexplored Territory quest. The following is taken from the Bigjra walkthrough. If I'm incorrect, you can look through the guide anyway for further information if you'd like.
"Fly on over to the Agate Circus, head to Route 2, then enter the Ametrine Mountain on top. From here, surf down the waterfall into 1F. Move the boulder from below on the left side, then climb the waterfall to cross the path from above. Dive down here to reach B1F.
"Once you cross the B2F ice puzzle, go to the top left corner of this room and dive down. Follow this path all the way to a New World Door!"
Saslen's post in Trying to find a legendary was marked as the answer