TWENTY years ago today, a small Texas company called id Software uploaded a file to an FTP server at the University of Wisconsin-Madison. That file was "Doom", one of the few video games that almost everybody has heard of. It sold in the millions, has been converted to run on almost any computing hardware available, and still boasts a dedicated community today, something virtually unheard of for something as ephemeral as a video game. What made "Doom" so special? The standard answers usually include the graphics, which were indeed jaw-dropping, at least for the time. "Doom" put its players into a convincing, three-dimensional world; it is hard to convey to someone who wasn't there at the time just how impressive an achievement that was given the hardware available. "Doom" required an Intel 386 processor with 4MB of RAM, which is to say about the same amount of memory as is taken up by a handful of images from a modern high-end mobile-phone camera.

The gameplay, which was excellent—tight, fast, trimmed of any fat—is often mentioned, too. So is the inclusion of relatively straightforward multiplayer, allowing two or more people to connect their computers together and see representations of each other in the world. Again, the impact of this is perhaps difficult to convey to modern gamers for whom such things are entirely routine. But "virtual reality" was a big buzzword at the time, and here it was, right on your desktop, you and a friend sharing a virtual world (and using it, of course, to blast each other to atoms).

The content itself was controversial. The game is famously bloody and full of satanic imagery, and it helped to start one of the earliest moral panics about the effects of video games on the young (the perpetrators of the Columbine high-school massacre, in 1999, were fans of the game). It was not the first game to give the player a first-person view (for one thing, id had already made another game, Wolfenstein 3D, that used the same viewpoint), but it was an early adopter. Nowadays, first-person shooters are some of the most profitable entertainment products ever made.

Yet for Babbage, the biggest innovation of "Doom" was something subtler. Video games, then and now, are mainly passive entertainment products, a bit like a more interactive television. You buy one and play it until you either beat it or get bored. But "Doom" was popular enough that eager users delved into its inner workings, hacking together programs that would let people build their own levels. Drawing something in what was, essentially, a rudimentary CAD program, and then running around inside your own creation, was an astonishing, liberating experience. Like almost everybody else, Babbage's first custom level was an attempt to reconstruct his own house.

Other programs allowed you to play around with the game itself, changing how weapons worked, or how monsters behaved. For a 12-year-old who liked computers but was rather fuzzy about how they actually worked, being able to pull back the curtain like this was revelatory. Tinkering around with "Doom" was a wonderful introduction to the mysteries of computers and how their programs were put together. Rather than trying to stop this unauthorised meddling, id embraced it. Its next game, "Quake", was designed to actively encourage it.

The modification, or "modding" movement that "Doom" and "Quake" inspired heavily influenced the growing games industry. Babbage knows people who got jobs in the industry off the back of their ability to remix others' creations. (Tim Willits, id's current creative director, was hired after impressing the firm with his home-brewed "Doom" maps.) Commercialproducts—even entire genres of games—exist that trace their roots back to a fascinated teenager playing around in his (or, more rarely, her) bedroom.

But it had more personal effects, too. Being able to alter the game transformed the player from a mere passive consumer of media into a producer in his own right, something that is much harder in most other kinds of media. Amateur filmmakers need expensive kit and a willing cast to indulge their passion. Mastering a musical instrument takes years of practice; starting a band requires like-minded friends. Writing a novel looks easy, until you try it. But creating your own "Doom" mod was easy enough that anyone could learn it in a day or two. With a bit of practice, it was possible to churn out professional-quality stuff. "User-generated content" was a big buzzword a few years back, but once again, "Doom" got there first.

The professionally cynical will dismiss "Doom" as merely another silly video game. Maybe. But the freedom to tinker that "Doom" allowed its players draws from a deepcurrent in computer culture. To the pioneers of personal computing, computers were not just devices for consuming other people's products. They were machines limited only by the imagination of their users, the means of production for the information age. Desktop computing was about liberation, giving ordinary people the tools to create things on their own (themes shared by some of today's 3D-printing advocates). John Carmack, the technical mastermind who wrote the game's code, is himself a hacker of the old school, committed to open-source software and hands-on experimentation. Playing around with the innards of a computer game was, for many people, their first steps into that world.