Earth-17 is a post apocalyptic parallel universe where an atomic war devastated the planet and where the survivors lead by Gardner Grayle, the leader of the Atomic Knights still fight to rebuilt their world. Known in the silver age as Earth-86, this earth was not only the landscape for the Atomic Knights adventures but also it was where the adventures of Hercules Unbound also happened.
Post Flashpoint, the Challengers of Unknown and a new version of the Justice League, the Atomic Knights of Justice had been integrated to the background of Earth-17, where the atomic war happened in the past, in 1963. This earth must not to be confuses with Earth AD (Earth-51), despite several times both earths had been depicted as probable futures for the main earth (pre-Crisis, Earth-1, post-Crisis, Earth-0), interchanging their order of succession.
Silver Age: Atomic Knights
The adventures of the Atomic Knights (published first in 1960) were set in a possible future after an atomic war ravaged the earth in 1986 (hence its denomination as Earth-86). In this world Gardner Grayle founded an order of the Atomic Knights to brings justice to the post apocalyptic world.
The adventures of Hercules Unbound are also set in this universe.
In 1983 however Earth-86 was retconned into an hallucination of Gardner Grayle, product of a virtual reality experiment than threatened to impose over the real world until Superman stopped the experiment. This adventure would be used to introduce the Atomic Knights into Earth-One continuity, and making all the previous appearances of the Atomic Knights into imaginations of Grayle. With the multiverse supression in 1986, this would be canon until 52.
Atomic Landscape: Blüdhaven
The background elements from Earth-86/Earth-17 had been used several times in the main earth, as a possible future. However the elements that include the Atomic Knights battle in a post apocalyptic wasteland were highly featured as an Infinite Crisis Aftermath, when Blüdhaven was destroyed by Chemo, becoming a toxic place. This Blüdhaven like Earth-17 was also used as pivotal point in several stories that continue through Final Crisis.
Earth-17 elements were also used along with the Kamandi possible future in several stories, implying that the Earth-17 like future preceded the Kamandi future, meanwhile in other stories it was the other way all along.
Post Flashpoint: Multiversity
The new version of Earth-86, rebranded as Earth-17 since 52 (the weekly), was more defined in Multiversity. Meanwhile most of the elements were kept, there were major incorporations, including the fusion of the Atomic Knights with the Justice League in the shape of the Atomic Knights of Justice, a team lead by Captain Adam Strange, an amalgam of Captain Atom and Adam Strange.
On Earth-17, the nuclear button was pushed in 1963 and nothing has been the same since. Fifty years after the fallout, Captain Adam Strange and his Atomic Knights of Justice lead an effort to preserve what’s left of Novamerika, while continuing to defend it. On a nuclear-infused planet where monsters, mutations and mad science have become commonplace, these adventurers know the fragile state of mankind sits on a razors edge between rebirth and heartbreak. As if channeling the famed Conquistador stories of old, this brave group of 21st Century adventurers is on a mission to seek out the elusive Cosmic Grail, the only weapon that can defend Earth-17 from a threat worse than any nuclear war—Darkseid the Destroyer.
Although the road ahead is completely unknown, the quest for the grail must endure in order to rebuild and defend this crumbled world and the still hopeful people who live upon it.
It must be add than elements of Arthurian legends are heavily featured in this universe, as the quest of the Cosmic Grail suggest.
Earth-17 inspired the same name universe used in the game Infinite Crisis. Is the home reality of atomic versions of Green Lantern, Wonder Woman, the Joker and Poison Ivy. At difference of their comic counterpart, they don't use any armor.