•Story Related Can’t Be Missed•
From when you’re first able to explore the map, you should have Selene’s portrait visible. She’s up north and West of prosperity and when you find her she’ll tell you she’s lost her field kit in a deep shaft and asks you to retrieve it.
It’s a good idea to have the ability to open safes as there are a few in this mission and it’s going to be difficult to return for them later. While you don’t need the lung capacity perk to complete this mission it will also come in handy.
Drop down to try to grab her field kit, don’t worry about the fall distance as the grating will collapse under you and you’ll be dunked into the water below.
Swim up and face south to see a doorway. Climb out of the water and through the tunnel towards the waypoint. There’s not too much loot in here but make sure you grab whatever you want and fully explore before using the valves. Once you do the area will begin to fill with water so run and swim back the way you came so you don’t drown.
Wait in the main area as the water rises. Then face west for your next doorway. Head inside and remember to explore and loot as much as you can before flooding the place. When you get to the larger room there are three feral dogs who will charge you so be ready for that too.
As you make your way towards the waypoint you’ll come to some flooded stairs - don’t dawdle or you can drown in this section. Swim into the water and make a right, then to the end and turn left.
Come around this corner and through the lit doorway and swim straight up. If your lung capacity is normal your drowning animation may kick in through this but you can make it.
Climb up the ladder and follow the corridor around to another valve, a red pipe, and a note. The note explains that if the valve is open the red pipe will explode if hit - so once you’re done exploring do that. There’s a baseball bat below the red pipe if you don’t have anything to hit it with.
This will smash open an entrance to another room with multiple valves. There’s another note here explaining the sequence of having every in valve open and every out valve closed. From the way you came in, you want to have the first right valve open, the first left valve closed, the next two valves on either side closed and the valve on top of the crates open. You can see which way the water is following by the marking on the pipes.
This will cause the water to flood so head out the exit. There’s rubbish in the way so you’ll need to wait for the water to catch up for you to swim over it. Follow the waypoint forward not turning off this main passage until you come to the end of the passage. Dive and go through the door and turn left and swim straight. Once you’re in the rounded section with the stairs you can swim to the top of the water to get some air before continuing.
Stand on the crate at the end of this corridor room and shoot or throw your melee weapon at the red pipe to your left. Then make your escape through the newly opened door.
Back in the main hole, you can see a ladder. Climb up it to the next doorway then turn right and left into the next room. Go up the stairs and head for the next lit section of stairs. There’s a snake in between yourself and these stairs so take it out first.
Head through the next room and in the red-lit room is a wolverine. Kill it and continue to the end of the room and turn right to go through the grated wall and down the stairs where another snake awaits just around the corner. Continue all the way around the central structure to find another snake before some more stairs down.
Go down and get into the water. The foreshadowed crocodile is in this pool and will attack you. Tap the button when prompted to fight and kill it then head through the door on the left.
Keep swimming towards the waypoint to the valve at the end of the room. Stand on the mound and turn the valve then swim back out the way you came in. If you’re swimming fast enough you can get ahead of the water at several points to take breaths then leave through the exit up the stairs.
The water will keep rising while you’re in the middle so swim close to the field kit so you can grab it on your way up. Then turn around and use the stairs behind you to get out of the hole. Talk to Selene and she’ll agree to come help out with healthcare at Prosperity.
Credit:
https://m.ign.com/wikis/far-cry-new-dawn/Deep_Dive