It is not underground.
The puzzle can be seen from out somewhere in the water.
If so, then notice where the south face of the obelisk is pointing.
Depart from the town dock in the boat, heading around the castle on the southwest corner of the island.
Around the back of the castle, move slowly and keep your eye on the green grass. At a certain point, the rocks will block just enough of the grass to make a perfect path.
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Complete spoiler:
This one is solved from the boat.
You might be able to solve it while riding the boat very slowly, but you can definitely solve it if you stop at just the right spot.
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Have you noticed that you can sometimes narrow down where an environmental puzzle is, based on its symbols location on the obelisk?
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Roll over each black area below for hints.
Each new hint gives away more than the previous one.
It involves the castle on the southwest corner of the island.
After you've sailed around the back of the castle, start looking at the greenery.
Map
Town Obelisk
South #3
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Intro
This one is not great, basically just visual.
It is not situated in the town.
This is roughly where you can find it:
Hints:
THE WITNESS - ENVIRONMENTAL PUZZLE HINT GUIDE