Your research into the location of the trove has proven fruitful, and you discuss how to proceed.
This quest is available if you have completed the quest The Darkness Below.
Starting the Quest
Talk to Alphinaud in the Lochs to begin the quest.
Speak with Alphinaud
Go northwest to where the orange arrow is pointing in the minimap. Talk to Alphinaud and there will be a cutscene.
Search the Ruins
Swim into the nearby water, then dive underwater by pressing Ctrl+Spacebar on keyboard or by pressing B on Gamepad. If it doesn't work, swim farther away from the edge of the loch and try again. Swim north to where the orange arrow is pointing in the minimap. Check on the sparkling destination near the glowing crystalline forms in the water.
Investigate the Passage
Swim east to the orange circle in the minimap. Check on the sparkling Stone Door.
The Drowned City of Skalla
Use the Duty Finder or Duty Support to enter The Drowned City of Skalla. Please see the The Drowned City of Skalla Guide for a walkthrough.
After you complete the duty in The Drowned City of Skalla and exit the area, there will be a cutscene.
Speak with Alphinaud Again
Talk to Alphinaud to complete the quest.
Next Main Scenario Quest
After you complete the quest, talk to Arenvald to accept the next quest: The Butcher's Blood.