Geothermal Valley - Climbing Down from the Cliffside Overlook

From the Cliffside Lookout Base Camp, where you re-entered the valley from The Acropolis and found a strongbox (6/8) with a HEAVY PISTOL PART (3/4)....

...grapple-swing over to the zip line and slide down to the ledge below, on the east side of the canyon.

Turn around and climb into the wooden shelter above.

Here you'll find a box of supplies and a document (19/33), On the Threat of Wolves from the Ways of the Valley series.

Climb down and use a grenade arrow to destroy the metal barrier on the wooden shack.

Inside is another strongbox (7/8). This one contains a BREAK-ACTION SHOTGUN PIECE (3/4).

Go up the steps beside the shack to a small, grassy ledge with a hardwood sapling, 3 salvage crates, and another document (20/33), Summer from the Pages of the Ledger series (Greek). There's also a survival cache (15/20) buried near the rock wall. (It will be marked on your map if you found Explorer Satchel #4 near the Aqueduct Ruins Base Camp.)

Climb the tree in front of the shack and jump over to the roof of the shack.

From there you can jump and grapple the rock ledge above the survival cache. Pull up to find a box of supplies.

Leap back over to the shack roof and from there to the tree branch, then the treehouse on the right.

Here you'll find an Archivist Map (7/9) that reveals the locations of numerous documents and relics in the area ahead. Slide down the zip line into the swamp below.