Screenshot by Gamepur
As you gather up all the Echoing Conches that will appear when the Midsummer Islands change in Genshin Impact, you will discover that some of them have secret quests for you. One shell will give you a quest called They Who Hear The Sea when you pick it up. This conch can be found on Twinning Isle, directly across from the spawn point.
Once you pick up the conch, a new quest called They Who Hear the Sea will appear in your world quest list. Tracking the quest will cause a waypoint to appear on a small flat island in the middle of Twinning Isle.