Move left to find a [PLANT], then head upstairs to view a cutscene.
Wade through the blood and head up the path until you reach some [ARROWS]. Use a burning arrow to (very briefly) take care of the gas up ahead, so quickly continue through the crack. You'll reach a new camp (The Pit), where you can find [DOCUMENT - #5 CONFESSIONS OF A SOLARI (IN HER HONOR)], as well as a [SALVAGE BAG] and [ARROWS]. [DOCUMENT - #6 LARA'S JOURNALS (NO ESCAPE)] can be gotten by using this camp.
All collectibles in the Geothermal Caverns area, part 1.
Move through the crack up ahead to reach the next hall. Aproach the waypoint to watch a short scene, then go down the walkway. A few steps in, look through the stalactites (and use Animal Instincts) to spot a [FIRESTARTER - #1 SACK] on the left. Shoot it with a burning arrow to check it off the list.
Head down and hide behind the debris until all cannibals are gone. You can find some [SALVAGE] just to the left of this small heap of flesh and bones. You may want to collect the [ARROWS] near the gate on the right; turning the valve here will release more gas. Shooting it with a burning arrow from a distance will clear the way. While this is the way out, there are some optional things to do.
• Climb the small shrine with some candles in the middle of the area (left of where the cannibals were eating) to find: [RELIC - #1 CEREMONIAL FANS (FUNERAL FAN)].
• The very first path to your left (southwest on your map) has [SALVAGE x2] at its end, but you'll have to kill a cannibal on the way back (a burning arrow will do nicely).
• The next path is blocked by some gas you'll first need to blow up, after which you'll need to fight another cannibal. Your reward is some [SALVAGE] and a [GPS CACHE] on the upper platform.
• You can find another [GPS CACHE] by following the walkway in the central cave.
• There's also the [FIRESTARTER - #2 SACK] in this walkway.
• Check your map and climb the northernmost ledge to find some [LARGE SALVAGE].
THROUGH THE GEOTHERMAL CAVERNS
After this, make your way through the gate on the east. As you walk up the stairs, immediately run to the right side of the large area and hide behind the large stalactite pillar as two Solarii guards approach. You'll also find [ARROWS x3] here to replenish your supply for now. When the first guard leaves, shoot the second in the head as he's taking a leak. Search the right wall for a [PLANT], an [XP BOX] and a [GPS CACHE].
Continue and hide behind the wooden fence to watch two Solarii guards play checkers. Something breaks and one of them goes to check it out (not in your direction), which is the ideal moment to take out both of them with headshots (the one closest first, obviously). [DOCUMENT - #3 ENDURANCE OFFICERS (REYES: DEAR ALISHA) can be found where they were playing checkers. There's an [XP BOX] in the upper right corner, and you can head up the stone steps leading back south. If you climb the wooden structure at the base of these steps you'll find the [TREASURE MAP] of the area. Turn the crank before reaching the top and fire a burning arrow into the gas to continue onward. Don't miss the camp at the altar at the very top, which contains some loot: [ARROWS x2], [SALVAGE], as well as [RELIC - #2 CEREMONIAL FANS (WEDDING FAN)].
All collectibles in the Geothermal Caverns area, part 2.
Climb over the box you kicked down with the explosion to continue. As you reach this walkway, a conversation between three Solarii guards starts down below. After the third one joins the two guards, igniting the gas close to them allows you to wipe them out with one swift move. You can find an [XP BOX] closeby, and right before you jump over to the walkway on the other side of the hall, you can also hit the [FIRESTARTER - #3 SACK] hanging near the door where the guards were conversing.
There's an [XP BOX] and a [GPS CACHE] at the right side of the end of the hall, on the lower ground. After looting the bodies, climb the walkway on the left side to reach a platform with a climbable rock wall and a [PLANT]. Head up and move to the end of the walkway; dispatch the unaware Solarii guard and burn the [FIRESTARTER - #4 SACK] hanging slightly to the right. Move to the right side of the walkway and kill the guard with a flashlight in the far back with a headshot. When the new recruits start chanting the "Flesh to the flame" nonsense, give them what they desire by shooting a well-aimed burning arrow in the heart of the gas leak. In the ritual area, search for [PLANT x2] (below where you're sitting) and some [ARROWS] (behind the fence from the left walkway you could've also taken). Don't miss the hidden area left of the altar; at the end lies some [SALVAGE] and [RELIC - #3 CEREMONIAL FANS (INSCRIBED SILK FAN)]. There's also [DOCUMENT - #7 DIARIES OF A MADMAN (LOST)] next to the sacrificial altar itself.
Head upstairs to find some [ARROWS], an [XP BOX] and a [GPS CACHE] next to it, as well as a valve for creating another gas leak. Open it up, then us a rope arrow to pull the cage closer. Fire a burning arrow at the gas (from a distance) and run through the walkway as the place collapses. After sliding down, quickly keep on running through the area filled with gas until you can climb up to the next area.
Here, wait for one of the Solarii guards to the left to scold the one firing arrows at Reyes (who's in the cage), then quickly take down that guard with flashlight on the right with a headshot. Wait for the other two to finish their conversation, and one of them will start fixing something. Take out the other one first, then him as well.
After the cutscene, collect [ARROWS] from your starting position, and some more on the main walkway. Turn the valve where one of the two guards was standing and shoot the gas to perform the first step in rescuing your crewmembers. There are more [ARROWS] underneath the cage (near a rock). The explosion has cleared the way, so go upstairs, grab some more [ARROWS], then jump over to the cage.
The next step is to shoot the gas left of the cage, immediately followed up by the second gas leak, so that the cage swings towards the counterweight. After this, scramble to the right side of the cage and consequently around and on top of it. From here, jump over to the nearby platform, break the two valves, take your distance and shoot them with a burning arrow to free the Endurance crew. [100 XP]. You can also find [ARROWS x2] on this platform.
Leap over to the yellow cages (quickly get a good grip on the second one) to reach safety [400 XP]. Make your way upstairs [100 XP] and open the double doors with your axe to reach the Solarii Palace, or what's left of it anyway.