To unlock this achievement, you need to deactivate the 10th orb. To find it quickly, load the 47th checkpoint (when you are underwater with the rotor tunnel on the right ). Keep diving down until you see a big door on the left wall. Open it by using the 2 red handles. Then, swim through the tunnel to reach the orb. (watch the video below):
Unfathomable achievement:Full walkthrough of the game: