From what I heard, many of the citizens met a very unpleasant end. The two heads of the cult were killed, but not before they mutated and crystallized several people in the very walls of the cavern.

[He shakes his head. Had he known it would have come to that, he would have acted much sooner.]

It's safe from the spread of the Calamity itself, so it could be used as a refuge. However, I'm loathe to make that kind of suggestion given its grisly and troubled history.