Mark Bristow writes for Bleeding Cool, and has a theory…
Is DeMatteis, Giffen and Porter's Justice League 3000 technically taking place on Earth 51?
Earth 51 was a world that Ray Palmer found where his friends and family were alive shortly before Final Crisis.
After the multiverse was reborn in Infinite Crisis, Earth 51 was shown to be an almost exact historical replica of the primary DC earth, but with a few twists. Most of the characters were in a happier place than on Earth 0, the primary DC earth. Their Ray Palmer had died in an accident, but his role was being filled by our own, having escaped the tragedy of his own life on Earth 0. Their Batman was in the darker place, having just lost Jason Todd. But, their League was happy and alive. Characters such as Ralph Dibny and Ted Kord never suffered the deaths that they did on the main DC earth in stories like Countdown and Identity Crisis. They were actually enjoying a semi-retirement with their families at the time of Countdown.
Earth 51 was the lighter-hearted late 80's reality that had not yet given way to the darker realities the 90's would bring, eventually leading to Countdown and Infinite Crisis.
This was until Superboy-Prime destroyed it out of spite and it was later restored by Nix Uotan, Earth 51's monitor.
Later, Final Crisis would become the "Great Disaster" that paved the way for Jack Kirby's Kamandi stories, which became the present setting of Earth 51, as it still exists in the New 52 multiverse, according to Multiversity.
This fits in line with the back-story Ice is telling in the most recent issue of Justice League 3000. She existed in a late 80's lighter-style DC Universe that was crushed by the Great Disaster and quickly became the "Kamandi Earth" from Jack Kirby's stories.
[Rich adds… someone asked JM DeMatteis something similar…]
@the_murci I don't even know what Earth-51 is! : )
— J.M. DeMatteis (@JMDeMatteis) February 12, 2015