As a point/particle. In games of the past which used voxels such as Outcast, Shadow Warrior and Delta Force, the voxels were not rendered as polygonal cubes. They were just solid color squares or, if you had an early GPU and the game supported it, circles. (See: Quake 2 particle effects).This works when the voxels are very small and densely packed but in Minecraft they’re enormous, about a cubic meter. So of course rendering them in the conventional way wasn’t going to cut it.