Earth One (DC Universe Supplement)
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The DC Multiverse is the continuity construct of the DC Universe. The DC Multiverse consists of worlds outside DC's main continuity. The number of alternate universes used by the Multiverse construct has varied over the years due to DC Comics' policy of using or abandoning the concept at various points in its publishing history.
After the publication of Infinite Crisis and 52, the Multiverse is again being used in print by DC Comics and consists of fifty-two alternate universes which are referred to by the numeric designations of the alternate Earths within them ("New Earth", "Earth-1", "Earth-2", "Earth-3", etc.). The numeric designation is used to distinguish the newer fictional Multiverse from the previous one, in which its alternate universes used alphabetic designations, such as "Earth-One", "Earth-Two", and "Earth-Three", instead.
The first parallel universe was discovered when Wonder Woman fell through a space-time warp and encountered her double, whose name, Terra Terruna, translated as Wonder Woman. After battling the villain Duke Dazam, Wonder Woman returned home.
The parallel universe was not visited again until Wonder Woman went to an earth which featured an alternate Earth where crime predominated. The second was "Magic-Land", an alternate Earth where magic, instead of science, was the dominant force in the world.
Barry Allen once used his powers of super-speed vibration to climb a rope suspended in mid-air and vibrates from Earth-One to Earth-Two where he meets Jay Garrick, the Flash of that Earth.
Each universe's Earth has its own set of superheroes, with their own unique characteristics and life histories.
Crisis on Infinite Earths
A cosmic battle erupted which ended with the recreation of the DC Universe from the dawn of time with a single universe.
One by one, a villain known as the Anti-Monitor destroyed several alternate universes. Heroes of the last five universes (Earth-One, Earth-Two, Earth-Four, Earth-S, and Earth-X) along with a handful of survivors from other universes (Pariah, Lady Quark, Alexander Luthor, Jr.) held off the destruction of the last five universes long enough to defeat the Anti-Monitor.
Those last five remaining universes were merged into a single universe with its own history that combined elements of all five, along with completely new elements. For example, there was a Flash named Jay Garrick who was a member of the Justice Society during the 1940s, and another Flash named Barry Allen was a member of the Justice League decades later, but there was only one Superman, who had a modified history, different in some respects from both the Earth-One and the Earth-Two versions.
Several pre-Crisis characters (most importantly the Kara Zor-El Supergirl and Barry Allen Flash) were killed during Crisis on Infinite Earths, and as a result were either erased from history (in Supergirl's case) or simply proclaimed dead (Barry Allen) in the new singular universe. Wonder Woman was thought to have been slain, but was revealed to have been thrown backwards through time, reverting back to the clay from which she was formed.
In Infinite Crisis, the Superman and Lois Lane of Earth-Two, the Superboy of Earth-Prime, and Alexander Luthor Jr. of Earth-Three, all survivors of the destruction of the original Multiverse, reappeared, and the former existence of the Multiverse was acknowledged. Earth-Two was recreated, and the surviving heroes who originated from Earth-Two were transported there.
In addition to this, worlds previously described only as "Imaginary Stories" or "Elseworlds" were revealed to be universes within the Multiverse, as shown by the presence of Superman Red and Superman Blue, Superman Jr. and Batman Jr., the Superman from the Elseworlds, and a world featuring Batman, Robin, and Wonder Woman in Aztec garb.
Eventually, Alexander's plan was circumvented when his equipment was destroyed by Superboy (Kon-El, a.k.a. Conner Kent), resulting in all Earths re-merging into "New Earth".
In 52: Week 52, it was revealed that the Multiverse was recreated at the end of Infinite Crisis with the creation of fifty-two separate Earths, separated by different vibrational frequencies and each with their own histories. These Earths were initially identical to New Earth until they were altered by the intervention of Mister Mind.
Contact between universes
Most inhabitants of the Multiverse are completely unaware of the other universes. The first character recorded to cross the gap between them was Uncle Sam of Earth-Two, who accidentally crossed over into Earth-X. Wonder Woman then crossed into an unnamed parallel Earth. Barry Allen, the Flash of Earth-One became the first recorded individual to visit another Earth, accidentally vibrating at just the right speed to appear on Earth-Two, where he met Jay Garrick, his Earth-Two counterpart.
Other characters with super-speed powers have been able to duplicate the trick, but it has not been done routinely. Magic and technological devices have done the job as well. The Justice League of America's "transmatter" device (ordinarily used to transport between their satellite headquarters and the ground), was pressed into service for annual events in which the League and some of their counterparts on other Earths faced a universe-crossing "crisis" of one sort or another. Wonder Woman's invisible jet was also able to vibrate her across the multiversal barrier, and she also crossed over when her magic lasso was struck by lightning. Superman could travel to other Earths at will while Captain Marvel used the magical Rock of Eternity that granted him access to any of the Earths.