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.
Go there and you will discover a Hilichurl digging, so take them out and then dig at the same spot. A chest will appear that is locked, and you need a code to open it. To get the code, open your event page and then go to the first section called Songs of a Distant Home. If you have all the conches collected, you will have a full section here and will notice something interesting about the entries.
They all begin with a number and give a four-digit code of 5-2-1-4. Entering this into the chest will open the lock and allow you to get at the loot inside. Just interact with the chest then enter the numbers one at a time and you will wrap up the quest.