What lingering DC mystery would you like solved once and for all?

I probably can’t even begin to formulate theories on most of yours, and can’t think of one of my own right now (all I can think of as far as mysteries go is the end of Doomsday Clock), but please submit some ideas!

What was the clock ticking down to that Firestorm saw? Trying to remember what this was in, but it never got answered from what I know.