Ground stop for all USA airlines

[puzzlegal]

This is being discussed in a breaking news thread

FAA issues nationwide ground stop due to "extraordinary circumstances" (for discussion, not breaking news)

This is only the second time since 9-11 that this has happened, and the explanation of “extraordinary circumstances and for reasons of safety” does not instill me with confidence that this is not something really, really bad. That said, I want to stay away from a lot of places online with conspiracy theorists going insane. Maybe this thread can be a repository for actual factual information about this.