From the ‘Aqueduct Ruins’ Base Camp, head a short distance to the northwest to find a slanted log in the water that leads to a tunnel above the far end. Climb into the tunnel and proceed inside. Upon reaching the first set of stairs, there are some Mushrooms and a chest of reagents to grab. Continue to follow the set path until Lara slides down a slippery slope and you end up in a pool below.
Swim to the ledge on the right and approach the small hole in the wall. Lara will climb on through. Use the roped up pillar here to create a zip line with the Rope Arrow attachment on the far side of the water. Climb across to the far side.
Move down the stairs and upon entering the circular room, look behind the skull pillars on the right side of the room for a chest containing a Relic [06/13 – Silver Basket] .
Climb up the ledge beside the Relic and follow the path until you reach a new base Camp ‘Aquifer Cavern’. You can use the Base Camp here to apply any upgrades you require and spend any skill points you have. Exit the Base Camp menu when you are ready to continue.
Move down the stairs into the Challenge room’s puzzle room. At the bottom, look behind the beached boat to your right to find a Document [14/33 - Greek] .
Around the room you’ll notice several spinning spools of rope and a boat with a rope pillar on the front. There is also a waterfall on the far right of the room and as such, the water is very fast flowing and if you fall in, Lara will almost certainly die, so we need to use the boat! Move to the spool of rope closest to the starting platform, stand next to it and use a Rope Arrow to shoot the boat to have it move to your location.
Jump onto the boat and after a few moments the rope will snap. Shoot the next spool of rope by the island in the center of the room. As you get close to the spool, have Lara cut the rope and as the boat starts moving, sprint and jump across to the stairs on the right side of the island. Climb the stairs to find a Document [15/33 - Greek] and two piles of Byzantine Coins .
On the far side of the small island from the Base Camp, you’ll find a third spool of rope and another on the wall on the far side of the water. Stand by spool number three and look behind the waterfall in the far left corner of the room to spot another boat. We’re going to have to act fast here and timing is very important. As such, I recommend you read the following paragraph completely before you attempt the next section.
From the spool on the island, use the Rope Arrow on the boat to bring it to your position. Now quickly jump across to the boat and as the rope snaps, quickly shoot the spool on the far side of the room (if you are not fast enough, you’ll hit the rocks and have to try again). Interact with the wheel at the front of the boat to wind it in until you are right below the handhold. Jump up when you are able.
Follow the path at the top of the stairs and in the first room you can loot a barrel of reagents, a box of reagents and an Archivist’s Map . Continue down the stairs to find an altar of sorts containing a Codex we can interact with to complete the Challenge Tomb [02/04] .
Before leaving, you can also find a chest containing a Relic [07/13 - Goblet] , Mushrooms and a barrel of reagents. Climb up to the ledge above the Relic/Mushrooms to find a weak wall. Smash this down to find a shortcut back to where we crawled through the small gap earlier.
Return back through the small gap in the rocks and swim to the far side of the pool. Use the pickaxe to climb the scalable wall here. Use the handholds to climb up and shimmy all the way to the left to return to the tunnel near the surface. Drop down and backtrack out of the cave.
Make your way back to the ‘Aqueduct Ruins’ Base Camp to move on.