Say Hi to the InternetG 10

NOT missable

When:
Any time after finishing the prologue, when you can first freely roam the island.

Method:
When viewing the map, the coordinates of your current cursor location will be displayed. Mark a custom waypoint at, then travel to, the following coordinates when free-roaming:

X: 620
Y: 558

Interact with the flashing head in the sand.

Achievement unlocked!