Mars (demo)
Tim Clarke's 1993 DOS demo that let you fly freely over a red voxel landscape — and sent thousands of hobby coders off to try the same thing.
Mars wasn't a group production from a party but a one-man job: a small program in which you steered a camera across an endlessly rolling landscape with sky and haze. The technique was the voxel landscape heightmap read column by column, the same idea behind NovaLogic's Comanche, except here it came in a file you could download from your local BBS and pick apart.
That turned out to be the point. Mars proved the effect ran on an ordinary 386 with VGA, and voxel terrain went on to appear in countless demos and intros through the 1990s, until polygon-accelerated graphics cards made the trick unnecessary.