Earth-42 has never existed in Marvel Comics. It’s probably always been there, seeing how in a multiverse, anything is possible, but audiences finally get to see what’s going on in a reality with no Spider-Man. Things aren’t great, as New York is on fire when Miles arrives, and the version of him that resides in this universe has taken on the mantle of Prowler. It’s a pretty bleak world, meaning it’s literally the opposite of the initial Earth-42 that exists in DC Comics.
Created by Grant Morrison and Gene Ha, DC’s Earth-42 was introduced as part of the New 52 run and is home to Lilliputian versions of the Justice League, which means they’re all little kids. They spend their time frolicking in a world without death or violence until a Superdoomsday from another reality arrives and kills Superman and Batman, leading to startling revelations.
Marvel has had a Prison 42 in the comics, which was used during Civil War by Iron Man and Mister Fantastic to imprison heroes in the Negative Zone who didn’t register their identities. It’s unlikely that Earth-42 in “Across the Spider-Verse” is meant to reference that, although one could argue that Miles is imprisoned there as he’s stuck there at the end of the movie, being held captive by the other Miles. Given the cliffhanger ending, it’s obvious Earth-42 will factor into “Spider-Man: Beyond the Spider-Verse,” so it isn’t the last time viewers have seen this new world.
“Across the Spider-Verse” is playing in theaters now.