Their appearance and whereabouts are unclear. We know very little about them, execpt that they were a VERY ancient and technologically advanced civilization, which had somehow come to ruin before the days of the Mannoths. Fragments of the structures they built, and the Amphiscus Orbs, are all that is left as evidence of their existence.
The Mannoth are old, but never even met any Amphisci. The fate of the latter is still unknown to this day.
It is said that the Amphisci arrived first in the Tunnel Dunes, slowly making the transition from the vast ocean to the humid soil under those ancient, dry arches.