Hidden in the water channel. Go back to the Aqueduct Control Room. Dive (tap the water). Serpent Key is under the grate.
You will see four colored valves: Red, Blue, Green, Yellow. The here is not random—it’s based on the mural above. The mural shows a setting sun: solution hidden escape lost temple chapter 6 best
For a visual step-by-step, you can refer to dedicated walkthroughs on community platforms: Video Guides Hidden in the water channel