It is not underground.
The puzzle is situated near the town dock.
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.
As you approach the castle, you will see two green trees. As you round the second green tree, you will see a start circle where the castle wall meets the rock, but don't use that. Instead, use the reflection of the start circle that appears in the water below.
Activate the start circle and then trace the path along the castle's dark brown molding / trim (whatever that brown line around the castle is called).
The path will continue as you ride around the castle.
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Complete spoiler:
This one is solved while moving.
Specifically, it is solved while riding the boat.
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.
As you depart from the town dock and head around the castle, look carefully for a start circle, or should I say, the reflection of a start circle.
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It begins near a dark green tree.
Map
Town Obelisk
South #2
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Intro
This one is fun! (And strongly related to the obelisk symbol above it.)
It is not situated in the town.
This is roughly where you can find it:
Hints:
You need the boat to be moving in order to draw the complete line.
This symbol is strongly related to the obelisk symbol above this one, so if you have already solved that one, consider how the shape of this symbol compares to that one, and that might be the only clue you need to solve this.
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General hint:
THE WITNESS - ENVIRONMENTAL PUZZLE HINT GUIDE