WARNING: This article contains SPOILERS for The Flash Season 4, Episode 9

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The Flash TV show keeps adding worlds to its DC multiverse, and the latest points straight at the evil Superman-Prime. Fans of Barry Allen and his teammates have made a habit of tracking the many parallel worlds and numbered-Earths uncovered thus far - both through Barry's powers and Cisco's breaching ability. In the past, it's allowed The Flash and Supergirl to team up, or call on a revolving door of Harrison Wells doppelgangers. But now, it's provided one of the biggest Crisis Easter Eggs since the show's premiere.

The bombshell was dropped when Flash fans least expected it, with attention turned to whether or not Barry Allen would be convicted of murder. But as the courtroom drama determined the future of the show's hero, a new metahuman put all of Central City at risk. The only solution was to unleash the danger on another Earth in the Arrowverse's multiple realities - one with a major connection to the most well-known version of an 'evil Superman.' Did you catch it?

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The Flash Nukes Earth-15, A "Dead" Planet

To make sure every fan of both TV show and DC Comics can appreciate how the writers slipped in this Easter Egg, let's add some context. The Thinker was clearly not content to just put Barry under the pressure of a possible murder conviction, making sure to turn another Central City resident into a walking nuclear bomb. That's not an exaggeration, either: the metahuman dubbed "Fallout" by Cisco elevated his radioactivity until he threatented to go nuclear, bathing the city in deadly radiation.

In the end, the team was unable to either find the metahuman or prevent this catastrophe from taking place. The solution, as it so often turns out to be, is for Barry to run around the walking meltdown as fast as he can to create a "vacuum" and funnel the radioactive fallout directly up into the sky. At which point the ball is passed to Cisco to save the day. All would have been lost if not for the quick thinking of Harrison Wells upon realizing that deadly radiation still needs to go somewhere. After a second's thought, inspiration strikes, as he informs Cisco of the ideal breach to create:

"Earth-15. Earth-15 is a dead Earth, breach it there now!"

The audience, like Cisco, is left to take him at his word. Not that there is any reason to question it, since... well, even for a version of Wells, bathing an unsuspecting Earth in nuclear radiation is excessive. And just as in the past, when random "Earth-Number" worlds have been dropped, nobody looks any deeper. The mutliverse seems almost infinite, so surely one version of Earth would have no life on it, right?

The issue for DC Comics fans isn't whether or not such a world exists, but why there's no life left on it to begin with. And when fans go digging into the actual fictional history of DC's Earth-15, their minds may be blown at just what DC Universe-spanning event and villain The Flash is teasing. A villain once known to comic book fans as Superboy-Prime.

Earth-15 is Dead Because Superman-Prime Killed It

While the namedropping of parallel, numbered worlds and universes in previous episodes of The Flash have been hit or miss - some are similar to the comics, others wildly different - Earth-15 is a definite hit. And yes, Harrison is right in saying that unleashing nuclear energy onto the planet would do no harm. Mainly because there's no way to harm a planet that... no longer exists. Despite the fact that at one time, Earth-15 was the most "perfect" universe in all of the DC Comics Multiverse.

That revelation came in Countdown #30 (2007), as part of the larger story leading up to the Final Crisis comic book event. For simplicity's sake we'll skip over the fact that the world was 'discovered' by Jason Todd, Donna Troy, and a Monitor named 'Bob' searching for a way to save the universe. Earth-15 was just one of several realities the team hopped between, learning that Earth-15 really was a best-case scenario for DC's heroes. Jason Todd became Batman having learned all of Bruce's lessons, Zod became an ideal Superman, husband, and father, and so on.

The group's stay on Earth-15 was brief, but the point was clear. Which made it all the more tragic when Superman-Prime arrived on this 'perfect' world not long after, in pursuit of his own idea of a perfect reality.

But with Superman-Prime, previously known as Superboy-Prime (and one of the most shockingly twisted and savage villains of DC's modern age) no possible world can ever be as perfect as the 'Earth-Prime' he once called home. Since it was written out of existence back during DC's Crisis on Infinite Earths, that leaves him with no real way to get home. Which is why he became as crazed and psychotic as he did, and why he must insist on destroying any world he finds that sets off his temper.

The heroes of Earth-15 did just that, and Countdown #24 followed Superman-Prime as he killed them one by one. First, it was Superman and his wife. Next came the Justice League, Wonder Woman, and Batman. But when Jason Todd used his last breath to assure Superman-Prime that the entire world would always hate him, the villain took an extreme step. Conceding that he would be hated, as he was on all other worlds, he flew straight through the planet. Tidal waves wreaked havoc beyond the impact itself, with the entire planet utterly destroyed soon after.

Superman-Prime continued on his mission from reality to reality, leaving Earth-15 nothing but rock in his wake. Eventually the DC Universe and Multiverse was rebooted, but Earth 15 remains a destroyed reality. Apparently, events have already led to its annihilation by the time of The Flash TV series. Is it a sign that the showrunners may adapt the evil Superboy/Superman-Prime now that Superman exists in DC's TV Multiverse? Or simply a winking bit of fan service to keep fans wondering what the next season of Arrowverse adventures may hold?

Either way, the damage has been done. Now we can only hope the showrunners make good on revealing who caused it.

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