Io

Following a serious nuclear attack on what was once the biggest city in the universe, the remaining livable regions in Io were converted into a meeting ground for meetings of peace agreements by officials from all over the system, referred to as the citadel.

Floating structures were made that could be taken in to the atmosphere in the event of another catastrophe, after a recent peace meeting, several hundred officials and attendees went missing with no trace. Investigations have been being carried out for months with no trace of the life that they're searching for.