You can see the original poster for Doom 2 buried within Doom Eternal's soundtrack. Yes, I know what you're thinking, and working this all out has nearly broken me.
Over on the Doom subreddit, there's a post that claims that you can see the poster for Doom 2 in the spectrogram for one particular track on the Doom Eternal soundtrack, composed by Mick Gordon.
This particular track is "Welcome Home Great Slayer," which you can listen to for yourself just below. I know what you're thinking: "How the hell is there a Doom 2 poster buried within that one track?"
I have tried working this out for myself, which led me down a whole rabbit hole of spectrogram analysis in music. I can't do a very good job of explaining this to you, so I'll let the excellent video below do the talking.
Even after watching the explainer above, I'm still convinced this is digital wizardry. I have absolutely no idea how Mick Gordon went about putting Doom 2's original poster into the spectrogram analysis for the track, nor do I have any clue how the original posted on the Doom subreddit managed to work out that there was a poster hidden in the track.
In not-so-magical news, Mick Gordon recently revealed that he wasn't able to mix the vast majority of the tracks on the official Doom Eternal soundtrack release. Lots of fans of Doom and Gordon's work are not pleased with Bethesda over this, as you can probably imagine.