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