O: Aqilat. Metal mining, Rule of Man ruins mysteriously abandoned.
As the prosperity of Aqilat increased, the ruling APCA turned their attention to the neighboring system of Nyarlathotep, then uninhabited. Initial surveys revealed promising deposits of various commercial minerals, and APCA launched a colonization attempt.
Only after the colony was well established did the belters discover a ruined settlement dating back to the Rule of Man. This appeared, like their own, to be a meteor mining operation; but it also appeared to have been depopulated several years after establishment. No explanation as to the cause has been succesfully advanced, and searching has revealed no clues as to what happened. An asteroid belt is a big place to search, however, and the answers may still be out there, somewhere.