Two chemicals, one feeding on the other, spreading and reacting throughout a solid volume. The same Gray-Scott model as the flat version, with each cell now exchanging with its six face neighbours instead of its eight flat ones.
Being volumetric changes what the patterns are rather than merely how they look. The spots of the flat model become spheres. The stripes become tubes. And where the flat version settles into a maze, this settles into a connected sponge with a real interior, full of passages that only appear once the renderer cuts into it.
That interior is the point. A flat maze has all its structure on the surface, where you can see it at a glance. A sponge keeps most of itself hidden, and you find it by looking in.
Reset seeds a small cube at the centre and lets the reaction spread outward.