1. You need the Enigmatic key, so story from V12 needs to be finished.
2. Go into the shrine underneath Aquamarine Cave (where Amber fell down), and dive in the little diving spot to the left. Save beforehand!! You have to dive at a specific point in that area, best would be as far down as possible, because when you get down you'll be inside of currents. Get onto the bottom tile so the current pushes you down to a spot behind a rock where you can't access otherwise. Go to the left and just follow the corridor until you get out of the ruin. Now there's gonna be a little building/tower in front of you, interact with it, it will say that a door somewhere opened.
3. Now go to the hidden area behind Aquamarine Cave. Enter the diving spot to the far right to reach the building which you can enter now.