It is not underground. It is out in the sunlight.
Map
Treehouse Obelisk
SW 1
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It is inside the castle (aka the "Keep")
You need to be high up to solve it.
Unlike many environmentals, it's not a solid color for the entire path but changes as you progress along the path.
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I'd HEDGE my bets you'll have a good chance of seeing it from near the laser within the Keep.
The start may be harder to see as it blends in with its surroundings.
Maybe see if you can find where the end is?
The environmental is made up of the walls of the hedge maze and the trellises over the gates. Starting point is Dark Green and in the cluster of trees beneath your feet.
The path progresses through the hedges in the following order: Dark Green, Brown, Light Green, Yellow.
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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.
This one may be more challenging as the picture on the obelisk isn't a line by line representation. There are a few more twists and turns that aren't depicted.
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It's aMAZEing how this is built into puzzles you'd likely solved earlier.
Intro
You can tell by the shape of this one that it's pretty fun.
Show hints
THE WITNESS - ENVIRONMENTAL PUZZLE HINT GUIDE