To complete Sunken Cavern: The Gouge in Horizon Forbidden West, you need to find all of the Greenshine Fragments in the cavern. This is the most complicated and difficult Sunken Cavern in the game, and you can’t actually complete it without finishing The Deluge side quest first, so do that. During that quest, you’ll flood The Gouge, and that’s what makes full exploration of the Sunken Cavern possible. Oh, and you’ll obviously need the diving mask for underwater breathing before you attempt this, or any other, Sunken Cavern.
There are two entrances to Sunken Cavern: The Gouge marked on the map, but for this guide we’re assuming you’re entering via The Gouge’s main entrance that you used during The Deluge quest. Go inside, rappel down, then drop into the water. On the south side of this chamber, a few meters below the water’s surface, there’s a sunken barrier. So, pry it open and swim on through. At the end of this long tunnel, swim to the bottom of the chamber to find the first Greenshine Fragment. You can also reach this chamber via the west entrance, and you may even have collected this Greenshine Fragment before completing The Deluge.
Swim back to the top of the chamber and take the tunnel to the southwest. At the end of this tunnel, turn left and squeeze through the crack at the northwest end of the chamber. Swim straight ahead to find the next Greenshine Fragment.
Related: How to get Greenshine in Horizon Forbidden West
Continue straight ahead to the northwest and you’ll encounter a Burrower. Swim past it as fast as you can, and find Greenshine Fragments against the north and west walls of the next chamber, and a Greenshine Cluster against its east wall.
Exit this chamber along the northeast tunnel, and find the final Greenshine Fragment at the end of it. Collect it to complete Sunken Cavern: The Gouge. Swim south, squeeze back through the crack. Swim east and up, then turn north. You can then turn left to use the west exit, or right to use the east exit.
