This puzzle IS underground.
Map
Desert Obelisk
NW #4
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You don't have to do anything special to "activate" it. You just have to find it.
You have to get inside the temple, then go down the first flight of stairs.
The part that's clever is how the 4th and 5th puzzles on this face of the obelisk are related to each other.
Hang out in that underground room tracing orange paths from right to left until you find them both.
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Complete spoiler:
Roll over each black area below for hints.
Each new hint gives away more than the previous one.
This puzzle is in the first underground room after the first flight of stairs.
That's about it! Find some orange things.
Show hints
Intro
The 4th and 5th puzzles on this face of the obelisk aren't especially "fun," but they aren't too hard either.
THE WITNESS - ENVIRONMENTAL PUZZLE HINT GUIDE