After taking the FIRST PIECE OF THE SCION and exiting the tomb, you'll emerge near the pool from the Lost Valley level. About halfway down, on the right (east) side of the pool, is an underwater tunnel. (No, it wasn't there when you played the previous level.) Swim inside to avoid Larson's bullets.
Continue forward through the narrow passageway...
...to a small room where you can surface to breathe...
...and pick up SECRET #3, a large medi pack (6) and magnum clips (7). (If you've found all of the previous secrets, you'll unlock the "Codex of Peru " achievement/trophy here.)
The final pickup of the level is only for the truly obsessed. Swim back to the pool, head for the right (north) bank, and climb out of the water.
Run past Larson (using a health pack on the way if necessary) and jump up and over the rocks, as if you were going to return to the valley. Larson won't follow.
Keep going to the steep wall of whitish stone, climb up...
...to find a large medi pack (8). You can't go any farther since the tunnel is now collapsed.
So return to the pool and either jump out and confront Larson (8)...
...or shoot at him from the sloping block near the cave opening until a cut scene ensues. (This apparently counts as a kill, even through it's really more of a K.O.)
DARK MODE: Even with brightness adjusted, dark images on a light background can be hard to see. May I suggest a reputable dark mode browser extension, such as Dark Reader? (Download only from trusted sites, like darkreader.org or your browser's web store.)
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