This area is also shown in a diagram included in the main walkthrough.

Surface and climb out of the water. Ignore the gunman in the room off to the side for now. Follow the ramp down to a room with tanks and cages containing various mutants.

Move the 2 green crates.

Underneath one you'll find Uzi clips.

Behind the other is a chemical analyzer. Place one of the MUTANT SAMPLES into this machine.

NOTE: If you missed secret #3, see the main walkthrough for notes on retrieving it now.

Place the other sample into the second analyzer in the corner to the right of the ramp.

This causes water to flow into a pool elsewhere.

Return up the ramp to the room where you entered the lab. Climb the stairs or pull up onto the ledge on the right.

Take out the gunman you saw earlier, and grab his small medi pack.

Follow the walkway around the edge of the pool. Climb onto the upper walkway...

...and pick up the rocket you spotted through the fence earlier. Pull up onto the small ledge above and get some grenades.

Then dive into the pool.

[Part 2 | Return to the Sleeping with the Fishes Walkthrough]