This puzzle is not underground.
Map
Desert Obelisk
North #7
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It is on a building or structure.
It is on one of the ruins around the desert temple.
You need to look at the puzzle while standing in the right position so that you see the sun's bright reflection on the entire path you are drawing.
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Complete spoiler:
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.
When you are at the obelisk and look at the desert temple, the ruins with the puzzle are behind it.
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