After solving the compass puzzle and opening the 3 doors, as shown in the previous section, go through the WEST DOOR first.

Follow the ramps down to a room with a stone ledge.

Climb onto the ledge...

... and hop down into the pit on the left side to get some flares. (The pit on the right is empty.)

Climb back up and drop through the open trapdoor into the pool.

This first flooded room has three tunnels leading out in different directions. The one to the west leads to a lever you can't use yet. There are also 2 trapdoors and a large pipe running along the floor.

Follow the south tunnel.

It leads forward then to the right...

...into another flooded room with a handle on the ceiling.

Position Lara below the handle and press Action to pull it.

This closes the trapdoor in the ceiling of the previous room.

There's also a small medipack on the floor in the northwest corner. Grab it.

Then swim back along the tunnel...

...to the opening in the ceiling of the first room to breathe. (Even though the trapdoor is now closed, you can still get air there.)

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