SOVIET INSTALLATION - ANCIENT CISTERN CHALLENGE TOMB
CHALLENGE TOMB ENTRANCE: As you explore this area you'll see a notification about a nearby challenge tomb. Again, exploring these tombs is optional, but the rewards are well worth the effort. To get inside, locate the log dangling from the crane. Shoot the hook block to drop the log through the planks below, revealing a circular shaft. Climb down the ladder into the CAVE below. Even if you don't fully explore the tomb, this descent counts toward the Into Darkness Challenge begun in the Installation Vista area. It is marked C2 on the annotated level map. (screenshots)
ANCIENT CISTERN CHALLENGE TOMB: Drop down several ledges, using Survival Instinct to spot a few magnesite deposits, and a mural (5/12). Lessons of Loyalty from the Byzantine Inscriptions series. Examine it for XP and Greek. Backtrack toward the ledges where you climbed down, then head to the right (south) through the rough archway flanked by primitive wooden totems. Follow the tunnel, dropping down twice as it descends. There's a relic (3/18), A Coin of the Realm from the Icons of Kitezh series, in an alcove just to the left of the next pair of totems. (screenshots)
The wall beyond the totems is climbable rock. Take a running jump across the chasm and latch on with your axes. Climb around to the right and downward. Move as far to the right as possible, so Lara leans out into the gap. Then jump and latch onto the next section of climbable wall. Climb down and around to the left. Then drop down onto the ledge below. Crack open 2 salvage crates. Then slide into the room below. There's another relic (4/18) in the dark alcove on the left. This one is a Golden Icon from the set called Icons of Kitezh. (screenshots)
Climb the stairs to a landing overlooking a small pool. Jump to grab the horizontal beam, activating a mechanism with 2 large gear shafts directly below. This mechanism lowers the water level but will also kill Lara instantly if she gets caught in it. Wait for the gears to turn so there is a gap between them. Then press B to drop into the pool below. Climb out of the water on the left and pick up up 2 Byzantine coins. Then swim across the pool and follow the passageway around to the right, collecting some cloth from the container in the corner. Take the arrows leaning against the wall and shoot the red oil can to blow open the wooden door. Wade along the flooded passageway to the Cistern Cavern Base Camp. (screenshots)
From the campfire, head down the stairs to the main room, a huge cistern. Lara spots her goal: a codex on a lectern sheltered by an ancient dome. Scramble up onto the elevated walkway, turn right, and move to the end. When you try to grab onto the rickety planks on the wall below the dome, they collapse, dropping Lara into the pool below. (screenshots)
Swim around the spike-topped fence and through the arched opening in the southern corner. Follow the passageway to a small room with a round, wooden door that has water leaking in around its edges. Pick up more arrows and a red oil can at the top of the steps. Toss the can onto the ledge by the door. (LT to aim, RT to throw.) Then shoot the can to blow open the door, flooding the room. Swim through the door you just destroyed and follow the passageway back to the main room. (screenshots)
Swim across the pool and climb the stairs. Here you'll find a bunch of arrows, a box of supplies in the raised alcove on the northeast wall, and several red oil cans. Pick up one of the cans and toss it onto the wooden platform floating in the far right corner of the pool. Now go up the stairs or scramble onto the ledge to the northwest. There's another raised alcove here where you'll find a container with cloth and a gold coin, plus a document (7/26) written in Greek: Correspondence from the Ancient Cistern series. Hang from the edge of this alcove and climb/jump along the ledges to the right until you can pull up into another alcove. This one contains a stack of gold coins and an Archivist Map (2/9), which reveals the locations of relics and documents in this area on your map. (screenshots)
Drop down from the alcove, climb the stairs, and then jump to grab the horizontal beam. Doing so opens a sluice gate off to the right. The rushing water sweeps the platform with the can across the pool, past the round, wooden door, toward the water wheel in the far corner. When the beam reaches its lowest point, drop down, draw your bow or pistol, and aim in the vicinity of the round door. As the water recedes, the platform floats back to the right. When it passes in front of the door, shoot the oil can to blow up the door. More water flows in through this opening, further raising the level in the cistern. The platform survives the blast and returns to its original position. (screenshots)
Swim through the opening where the round door was to get back into the side room. With the higher water level, you can now climb out on the ledge near the huge, metal gear shaft. Move around to the stairs on the right and jump across the gap to reach the higher ledge on the other side of the room. There are two raised alcoves here. One contains a box of supplies, the other a document (8/26), A Warning from the Ancient Cistern series. Examine it for XP and Greek. (screenshots)
Notice the round, wooden door. It is similar to the others, but there are no red oil cans nearby to destroy it. You'll have to return to the main room to get one. Follow the passageway on the right side of the room (when facing the wooden door). Pick up some salvage behind the ornate window grate. Then break down the crumbling wall at the end of the passageway with your axe. (screenshots)
Swim back across the big pool to the ledge with the oil cans and toss one onto the wooden platform, just like you did before. Climb onto the ledge above and jump to grab the horizontal beam. Again, this opens the sluice gate, sweeping the platform with the can to the left. This time, instead of trying to shoot the can, drop down, swim across to the platform (hold B/Shift to swim faster), and climb on top of it. Then, as the platform floats back to the right, toss the can up into the opening you made with your axe. If there's still time, scramble up the wall into the opening. Otherwise, swim back to the ledge, pull down the bar, swim to the platform, and try again until you're able to get both a can and Lara up into the opening. (screenshots)
Carry the can into the side room. Go up the steps and around to the end of the ledge closest to the wooden door. Toss the can onto the doorsill. Then shoot the can to destroy the door, raising the water level to its highest point. (screenshots)
Swim through the opening where the wooden door was to get back to the main room. If you missed document #7 or the Archivist Map earlier, you can swim across to the alcoves on the northwest side of the pool to get them, along with a few Byzantine coins and some cloth. Finally, swim to the dome. Pick up some salvage and more cloth, then examine the codex to automatically learn a new skill: Natural Instincts. This reveals the location of nearby crafting resources on your map and causes them to glow when you approach them. You'll also receive 10,000 Expedition credits if this is your first time raiding this tomb. (screenshots)
When you're ready to go, swim back to the passageway in the northeast corner and follow it back to the Cistern Cavern Base Camp. From there, you can fast travel to any other camp or continue along the passage to climb out the way you came in.
Pick up the main walkthrough near the CHALLENGE TOMB ENTRANCE, just north of the SAWMILL.
†UPDATE HISTORY: 11/29/15 - First draft of walkthrough posted online.
1/28/16 - Added PC keyboard controls and made a few minor tweaks.
4/24/16 - Made a few minor tweaks, including switching the order in which the two documents are collected.
7/16-17 - More tiny tweaks: document #7 and the Archivist Map are now retrieved before raising the water level.