The Drowned Hopes side quest in Horizon Forbidden West can be started by talking to Corend at Camp Nowhere, northeast of the Las Vegas ruins, but you can skip the first part of the quest by starting instead at the Carja Camp southeast of Plainsong. Talk to the two Carja Scholars there and they will tell you that they want to return to their dig site. You — being the hero and everything — volunteer to help.
Related: How to complete the Shining Wastes Tallneck in Horizon Forbidden West
Follow the marker to the dig site, Jagged Deep Delve, on the far side of the lake, and kill all the machines you find there. The Scrappers are easily disposed of, but the Widemaws are much tougher and more dangerous. Place Purgewater traps in their path, and use any other Purgewater weapons or tools you might have — they’re strong against all other elemental attacks. Otherwise, target their bellies, butts or, if you get the chance, the inside of their mouths.
For the next objective you’re going to need to do some underwater breathing, which means that, if you haven’t done it already, you’re going to have to start the Seas of Sand main quest in order to craft the diving mask Special Gear. Swim out into the marked area until you discover a Sunken Wreck, then dive down to investigate it. Use your Focus to highlight a Security Console inside the wreck, then swim down to the bottom of the northwest side of the wreck, and swim through the gap in the rocks.
Examine the Security Console, then search the Ancient Trunk to find the first of three Vault Keys. Follow the marker to discover Dread Bluff, the site of a second Security Console. Climb inside the ruin, drop into the water, then climb back up to the pink-lit hatchway. The second console is in the room beyond. Once again, use the console to open the trunk, then take the Vault Key out of the trunk.
Go back the way you came, and back up to ground level using your Grappling Hook. Follow the marker to Devil’s Slide, the site of the third and final Security Console. Swim out to the marked area to discover a Sunken Barrier. Dive down, pry open the door, and enter Devil’s Slide. Follow the marker around to the left and pry open another door to find the third Security Console, and third Vault Key.
Swim back out and follow the marker to the Vault, which is submerged in some underwater caves. Pry open the rocks, swim through the gap, then turn left and swim to the surface. Follow the marker up the bank and through the small canyon to find the ruins of a facility guarded by a Snapmaw and two Apex Leaplashers. Deal with the machines however you wish (we snuck past them) and go into the ruins.
Insert the three Vault Keys into the three locks, then go through the open door and pry open the cache. Search the Ancient Trunk to get the Mobile Cover System Prototype, then return to the Carja Scholars at the Carja Camp. Talk to them, then use their workbench to craft the Guardian Tripcaster and complete the Drowned Hopes quest.
