Witcher 3 Hidden in the Depths Walkthrough
On the island of Faroe, the southeastern of the main islands of the Skellege Isles. This can be found on a beach to the southwest of the ‘Harviken’ fast travel marker.
This location of interest will pop up as a Hidden Treasure marker as you get close. As you approach it, a Endregea Worker will attack you. Dispatch the monster before using Geralt’s Witcher Sense to examine a corpse sitting nearby, close to a cave entrance.
Upon looting the body you’ll receive a Message and a Scratched Key. Read the Message item in the Key Items section of the inventory. This will trigger the ‘Hidden in the Depths’ side-quest. For some reason, this will also automatically complete.
Ignore that however and head into the nearby flooded cave entrance. At the back of the initial cave, enter the pool of water. Dive under and swim to the next pocket of air ahead to reach a larger cavern area.
Defeat the Venomous Arachas and 4-5 Endregea Drones who surface and move in to attack you. After clearing the area, make your way to the far, eastern section of the cavern area to locate a treasure chest which will glow red with Witcher Sense active.
Use the Scratched Key on the chest here to complete the side-quest. It contains a Runestones, Glyphs and some assorted crafting goodies.