In the SUB-01 level:
1) Open the back of the GNOG (interact with all four of the mechanisms until all the fish light up)
2) Drain the water (click on the weird head thing at the top left corner several times)
3) Flip back to the front and slide both of the eyebrow-looking bits. A fish-looking device will open it's mouth and start "singing".
4) Flip to the back. The knob at the bottom right corner should be interactable now. It raises and lowers that diver you see. Spin that knob to make the diver dive as low as possible. You'll know you've gone far enough when the screen shows something that looks like the icon for this achievement.