Climb over the stacked boxes blocking the tunnel to the south (i.e., just beyond the hole where you found the last salvage crate).
Behind this stack you'll find the Research Base Treasure Map, which reveals the location of all major collectibles on your in-game map. Climb back out the way you came.
Proceed toward the opposite end of this same hall, picking up some rifle/SMG ammo, prying open the salvage locker on the left, and opening the food cache near the stacked boxes. Then climb up onto the boxes piled against the right wall.
Crawl over the boxes and drop down into the space behind the crates.
Here you'll find the final GPS cache (3/3) in the level and the game. See the note in the main walkthrough about the significance of the last cache. Climb back over the boxes.
Return to the "T" intersection with the black-and-yellow-striped turntable set into the floor. Turn right (west) hear and continue across the wooden bridge into a large cavern.