New Earth (also known as EARTH-0) for a time was the main and central DC Universe. This earth was remade twice.
The first version of this Earth was created as a result of Crisis on Infinite Earths. It was composite of the pre-Crisis Earth-One, the pre-Crisis Earth-Two, the pre-Crisis Earth-Four, the pre-Crisis Earth-S and the pre-Crisis Earth-X. After the events of the Crisis, it is shown that the characters who entered the "heaven" do not end up very pleased. Their new realm was crystal-like and responds to the emotions of the inhabitants, Superboy-Prime, Alexander Luthor, Superman of Earth-Two, and Lois Lane of Earth-Two.
The Multiverse is merged again, and a New Earth is born out of it.
It's also revealed that they have been monitoring the heroes of New Earth as they descend into darkness, and believe this generation unworthy of their legacy. They return to bring back the Multiverse, in an effort to restore the idealistic values of their own time. Their ambitious plan resulted in: The infinite Earths, which had collapsed into a single world during Crisis on Infinite Earths, diverged again into multiple Earths only to collapse back into a single "New Earth" with a slightly altered history. New Earth was now a composite of the pre-Crisis Earth-One, the pre-Crisis Earth-Two, the pre-Crisis Earth-Four, the pre-Crisis Earth-S, the pre-Crisis Earth-X, and the Dakotaverse.
Futhermore, Lois and Kal-L did not this crisis, and Superboy-Prime was imprisoned. A short time later, Alexander Luthor was electrocuted by the Joker, and then shot in the head by Lex Luthor.
Also, from 2006–2007, a limited comic series titled 52 established that a new multiverse now exists, with Earth-0 as the primary Earth. Unlike the original Multiverse, which was composed of an infinite number of alternate universes, this Multiverse is composed of a predetermined number of alternate universes, which were originally referred to as New Earth and Earths 1 through 51. New Earth is instead designated Earth-0.