This puzzle is not underground.
Map
Desert Obelisk
North #1
>
>
It is harder to solve than to find.
It is on a building.
You need to look at the puzzle while standing in the right position so that you see the reflection of the sun on the golden parts of the puzzle.
Complete spoiler:
This is a good place to stop reading. You now know roughly where it is...
Roll over each black area below for hints.
Each new hint gives away more than the previous one.
It is on the front of the desert temple.
The whole puzzle has to look the same color in order to work.
Think about the core mechanic of the area you are in.
You have to use the sun.
Show hints
Intro
This is not the most fun one but it is certainly not boring.
THE WITNESS - ENVIRONMENTAL PUZZLE HINT GUIDE