It is not underground. It is out in the sunlight.
Map
Treehouse Obelisk
SW 2
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It is inside the castle (aka the "Keep")
You need to be high up to solve it.
It is relatively close to the hedge mazes.
Looking out over the hedge mazes from by the Keep laser, look down and to the right. The environmental puzzle is a brown dirt path leading from the yellow hedge maze.
It starts by the exit to the Yellow Maze and ends in the doorway up the stairs.
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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.
Intro
This one should be fairly easy to spot.
Show hints
THE WITNESS - ENVIRONMENTAL PUZZLE HINT GUIDE