Over the course of a couple of months this year I learned path tracing, signed distance fields, GLSL coding, and compacting JavaScript using a coding style you should never use in real life. But this all resulted in a system that can create "photo-realistic" 3D images of an otherworldly scene in 16000 bytes, no dependencies.

Monte Carlo path tracing is a very interesting technique, albeit very slow to render. It still amazes me how you can get a huge amount of variation and realistic lighting from a tiny amount of code that basically simulates millions of light rays bouncing around in a mathematically defined scene with some randomness applied.