In Zero Hour, Hal Jordan had gone insane, killed the Guardians and stolen their power, and was trying to undo the effects of the Crisis; that is, he was trying to recreate the Infinite Earths. The Zero Hour story finished with Hal being defeated; but it was shown later that he didn't die as he appeared to. And sometime later, apparently not connected to the above, Hypertime we introduced to the DC Universe. Sooooo... some fans have speculated that Hal actually succeeded in undoing the Crisis, and thus Hypertime now exists.
DC has not admitted to that, so far as I know! ;)
That makes my head spin!
The only DC multiple Earth I was aware of before I read "Crisis" was Earth-2. Of course there were also the series set in future history, like the Legion of Superheroes in the 30th century, which is another complicating factor. Plus they tried to integrate stuff from non-DC series, like bringing Captain Marvel in.
Incidentally, I'd need to look up the link, but there's currently a project trying to construct a timeline inter-relating all the various Marvel series, i.e., what was going on with the FF during the events of "Avengers" #100, etc. It's a huge labor but pretty interesting.