Once you've taken care of both crocodiles, explore the ledges along the sides of the room to get a small medipack a box of shotgun shells...

...and more shotgun ammo.

The opening in the floor leads to a passage with bars blocking the way. The trapdoor next to that opening is closed.

To open the trapdoor, swim up underneath the overhang on the east side of the room...

...position Lara beneath the lever and press Action to pull it. This opens the square trapdoor on the bottom of the pool.

Swim down through the trapdoor...

...then forward (west) along the bottom to the square door. Face the gray tile in the in the middle of the door and press Action to open it.

Return to the surface to get air and save the game. The next sequence requires some skillful swimming, and it's easy to get disoriented and drown the first time (or three). When you're ready, swim down through the trapdoor...

...then straight on through the square door you just opened. Continue along the passage as it turns left.

Swim through a narrow, diagonal gap...

Straight on through a passageway with a low, curved ceiling. Then turn right.

Swim straight through a section of the passage with uneven floor and ceiling. When you reach the far wall, swim up to the left.

Continue forward up the next slope. Here the passage splits and you can go upward or right. Turn right.

Lara seems to want to get stuck in this narrow, uneven opening, but I found that if you try and keep to the left, she'll usually make it through. At the end of this passageway, swim straight up...

...into a small room with air and a large medipack.

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