This sequence is indicated by the broken pink line on the diagram in the main walkthrough.
Stand at the tunnel opening facing south (switch behind Lara). Jump into the water and swim down.
Halfway down the shaft you'll pass an opening on the left (east). Ignore it and continue to the bottom.
Pass the hand symbol at the bend and continue forward (south).
A little farther along you must turn left (east) as you follow the tunnel.
At the top of the next slope is the tile with the snake symbol. Here you can either continue forward (east) or upward. Swim up...
...to surface in ROOM #3.
Pull the switch to open the trapdoor to the next room (#4 in the diagram).
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