Half-Life 2 was released by Valve in 2004 and built on the Source engine, whose reusable sound library became familiar far beyond the main campaign. Its door effects were carried into custom maps, Garry's Mod creations, machinima and Source Filmmaker videos, where builders needed believable futuristic mechanisms. Because so many players spent hours opening identical doors in City 17, the short slide and clunk became instantly recognisable shorthand. It remains popular in edits and game projects as a clean, nostalgic piece of sci-fi foley that needs no explanation.