Since the originalwas released as open source code in the late '90s, hackers and modders have taken great joy in porting it to everything from Android Wear watches to printers . Now, those efforts have reached what may well be their zenith, with the release of a new mod that allows you to run a copy ofinsideitself.

OK, if we're being technically accurate, this is actually Doom running inside GZDoom, a heavily modified Doom source port that was first released in 2005 to bring a slew of modern gaming features to the 1993 original. The author also warns that the in-game versions of Doom and Wolfenstein 3D available in the mod are only "semi-complete." Still, the sheer amount of near-pointless effort and dedication needed to get GZDoom to run what is essentially a version of itself within itself is impressive (and kind of frightening).

Porting Doom to GZDoom was made possible through some elegant work on Action Code Script (ACS), a tool first introduced to the Doom engine in 1995's Hexen. ACS was designed to allow modders to create more interactive environments through simple bytecodes that did things like open doors, play sounds, or move items and characters around in response to player actions. The basic bytecode-based language in that game was later extended in the ZDoom source port to allow for high-level programming features like named scripts, functions, arrays, and entire libraries. Those additions made their way into the later GZDoom as well.

Modders have been using ACS to create mini-games within Doom since at least 2004, when Doomworld founder Andrew "Linguica" Stine created the almost self-explanatory "Doomtris" and "Doom Racer," which could be played in special arcade cabinets in the game world. Since then, there have been ACS Doom ports of arcade classics like Arkanoid and Pac-Man, and also simple original games like "Super Turbo Turkey Puncher 3" (which delivers what it says on the tin quite well, all things considered).

So the next time someone tries to impress you by showing how you can play a version of Minecraft running inside of Minecraft, you can tell them that the idea of running nearly arbitrary code within a game environment isn't just for millenials. Doom modders have been doing that stuff for over a decade, and they don't show any signs of stopping now.