Prepper Stash: Side Effects
Start this one at the Bright Warden Radon Spa, east of the Hope County Jail in Faith's region.
The note is on some long crates in front of the mine entrance.
The stash door is on your right as you enter the mine, but you can't open it yet. Instead take the left (east) tunnel to a barricade. Do make a point of remembering your journey through here, as you'll need to retrace your steps later in the dark.
Blow it up with dynamite, but remember to keep your distance (same goes for any companions tagging along). There's a valve in the next room, but you can't use it. Instead go west, following the tunnel to a room with another barricade and a green door.
There's nothing behind the door yet, though you can use the room for cover when you blow the next barricade.
Proceed west. The mist hides a drop in the floor here, but it's more of an inconvenience than a danger.
Past that is a slatted barricade you can bash through manually.
Pass through a narrow doorway, then crouch to enter a small passage.
Keep to the platforms on the left (east) wall to avoid the drop in the next room.
Jump to the far doorway and pass through. Down this tunnel you'll find a blue machine with a yellow switch. Throw it to get the vents working, sucking all the Bliss from the air.
Now it's back the way you came, only with worse lighting, and rabid Angels on the loose. Two are in the room with the easterly platforms.
Up the stairs from the slatted door, a 'dead' Angel will remind you about their resurrection abilities.
Use your grapple to cross the gap. There'll be an Angel in the green door room, and two more down the passage.
Those should be the last hostiles you have to deal with before getting back to the door.
Enter to find the stash, and complete the location.