The latest episode of The CW's The Flash, "The Trial of the Flash," was largely about, well, the trial of the Flash. But there was also a metahuman subplot involving Fallout, a meta who is a walking nuclear bomb. While the resolution of that B-plot — wherein Barry funnels Fallout's radiation upward so Cisco can vibe it to another dimension — might have seemed simple and resolved, in actuality, Harrison Wells may have dropped a major Easter egg for where The Flash (and the Arrowverse) could be going in seasons to come as a result of that seemingly innocuous moment: Earth-15.
When Cisco asks where he should vibe all that intense radiation, Wells tells him to send it to Earth-15 because "Earth-15 is a dead Earth."
And that might prompt you to wonder, "why is Earth-15 dead and what was it like before that?" Considering "the multiverse" is the watchword for both The Flash's stories and the Arrowverse as a whole at this point, those are questions worth asking.
Thankfully, the answer to those questions exists. At least, they do in the world of DC comics.
WHAT WAS EARTH-15 LIKE?
Interestingly, prior to it's total annihilation, Earth-15 was kind of a paradise. Crime was virtually non-existent on Earth-15 and the superheroes that remained were largely inheritors of their respective names rather than the originals. The sidekicks and protégés had become the masters.
On Earth-15, Wonder Woman is Donna Troy; the Batman mantle has been taken up by the second Robin, Jason Todd; rather than Kal-El, the Superman title belongs to Zod. There's also a Kyle Rayner-like Green Lantern and a female version of The Atom, Jessica Palmer. They all work together to keep the peace and are, so far as we know, more or less perfect at their jobs.
But then Superman-Prime shows up and literally destroys the entire planet.
WHO IS SUPERMAN-PRIME?
Oh, gosh. Where to begin? Here is the short-ish version:
Superboy-Prime (wait for it) is a version of Superman from a place known as, surprise, Earth-Prime. And Earth-Prime is, in many respects, our Earth, aka the Earth where DC Comics is real and the comic characters it creates are not.
Funnily enough, Barry Allen accidentally travels to Earth-Prime at one point and enlists the help of real-life DC editor, Julius Schwartz, to create a cosmic treadmill to get back to Earth-One.
But that is not the only way that Earth-Prime diverges from our actual Earth and connects with the DC Universe we know and love. There's also a young man who lives there and his name is, as you might have guessed, Clark Kent. But this Clark is not from Kansas nor is he an intrepid reporter or a superhero.
He is, however, a fanboy. Despite the jokes his friends make about his name, this Clark does like the character of Superman. And, wouldn't you know it, he gets to meet the Earth-One version of Superman, which results in Superboy-Prime finding out that he is from Krypton and is a version of Superman. Earth-One Superman takes Superboy-Prime with him to multiverses unknown so he can be taught to be a hero.
Then Earth-Prime and most other Earths are destroyed by the Anti-Monitor (long story).
The short of it is, Superboy-Prime grows up, becomes Superman-Prime and is very bitter about losing his world. He eventually becomes so powerful and so bitter that he starts going to other Earths to kill other versions of Superman (kind of a Highlander situation) and also maybe destroy those entire Earths, too, in the process.
And that, in a nutshell, is who Superman-Prime is. And also what happens to Earth-15.
WHAT DOES ALL THAT MEAN FOR THE ARROWVERSE?
Potentially? A whole lot.
Here's what we know right now: Barry and Cisco just fired a LOT of radiation to an Earth they were told is 100% dead.
But is it? In the comics, at least, the Earth-15 versions of Green Lantern and Jessica Palmer, aka The Atom, escaped the planet before it was destroyed by Superman-Prime. So there's no reason to assume that the Arrowverse couldn't also have an Earth-15 where there were some survivors.
And that's not all. One of the reasons Earth-15 is discovered in the first place is because some other Earth DC heroes are in search of a hero we know well from the Arrowverse -- Ray Palmer. Ray is currently on DC's Legends of Tomorrow, which is largely about time travel. So there's no reason that, in addition to traveling in time, Ray and the Legends team couldn't also travel to a parallel Earth, such as, say, Earth-15. Barry and Cisco do it on the reg.
And then there's Supergirl's elephant in the room, Superman. People love Tyler Hoechlin's performance, but he's seldom used because that would detract from the heroism of his cousin, the title character, Supergirl. He might help Kara fight Reign on Supergirl or we might see the supposed fight between Superman and Eobard Thawne that was mentioned on Legends of Tomorrow.
Or, taking into account our Earth-15 theory, Hoechlin could be repurposed on The Flash as Superman-Prime.
Together the heroes of the Arrowverse have taken on aliens in the Dominators and they've fought multiverse Nazis -- but they've never faced anything like a super-powerful (and super-evil) version of Superman.
And none of that even touches upon the Cosmic Grail, a supposed fragment of Earth-15 that holds enormous powers, possibly from the home of the Lantern Corps, Oa.
Harry has name-dropped Earth-15. There's established DC canon for Ray Palmer to go to Earth-15. And given how Barry travels to alternate Earths by accident sometimes, it's not crazy to think we might be introduced to Earth-Prime at some point, thus also opening the door to, survey says, Superman-Prime.
If a show like Supergirl can financially manage introducing the Legion of Superheroes, there's no reason The Flash (and potentially other shows within the Arrowverse) couldn't encounter an alternate Green Lantern or a Superman-Prime.
And if that all sounds far-fetched, just remember: the show's writers could have named dropped any place for Cisco to filter out Fallout's radiation, but they chose Earth-15; The Flash is all about name-dropping things in an intense moment only to bring them back later. After all, that's how we got to current big bad DeVoe, right? Right.
What do you think? Would you want to see some part of the Lantern Corps introduced into the Arrowverse? Are you hopeful for a version of Superman-Prime who doesn't say the cringeworthy phrase, "I'll kill you to death!" Fingers crossed.