Crisis on Infinite Earths #1 of 12. This is the series that the current Countdown to Infinite Crisis in the DC Universe is alluding to. For fifty years, DC was mucking up their own continuity. That's because they were trying to merge titles into a universe and that hadn't been done before. So when Flash was reading a Superman comic, he couldn't just live in the same world as the original Superman. So they invented alternate realities and Earth-1 and Earth-2 superheroes. It got really confusing. In fact, when continuity buffs would throw a question out, DC would come back with something like, "That was the Earth-2 Superman." Before the popular Elseworlds comics, DC would throw in "imaginary stories" of characters so they wouldn't have to mess with continuity. But it started to grate on the nerves of fans and creators alike. So they decided to destroy the whole thing and start over. They first did it with Batman and the re-envisioning of his origin. DC basically destroyed all of the other universes and kept only one. This series had many epic moments that have rattled the DC Universe for the past 20 years. Even though they basically started from scratch popular characters like Superman and Batman, other characters first appeared in this series, like Kid Flash Wally West finally becoming the one and only Flash. I think that's why DC is starting again with this Infinite Crisis. They still end up referencing events that happened before this series in 1985 when it was supposed to erase all that.