The sound comes from the tripods in War of the Worlds, the alien war machines first described in H. G. Wells' 1898 novel. The 2005 Steven Spielberg film gave them a signature deep horn blast used before attacks, and clips of that call spread online as a shorthand for a massive alien threat arriving. Editors lifted the short blast for soundboards and edits, where it now signals an invasion, a giant machine waking up, or anything impossibly large approaching off screen.