I just finished watching a live-coding session (from 2 years ago) by Inigo Quilez (ex Pixar), where he constructs an entire ray-marched landscape using only maths (mostly SDFs) in pure GPU shader code.

The stream: https://www.youtube.com/watch?v=BFld4EBO2RE

The demo: https://www.shadertoy.com/view/4ttSWf