You must locate the communication center in this level to radio up to the Terran mothership. Exit into the room straight ahead of you and kill all the aliens. Walk around the water and pick up all you need without enter the room with the closed door. Look down into the water and you’ll notice a crack in the floor. Dive in, crouch into the hole, and turn left. Go forward quickly until you see an opening above - this is a secret area with ammo and health.  Jump out and pick up these goodies and you may also enter the elevator to the secret level.