Comment by Kim_Bruning

It's a game of life simulation that is -itself- running a game of life simulation that is running a game of life simulation that is ...

https://en.wikipedia.org/wiki/Conway%27s_Game_of_Life

Game of Life is Turing-complete, and has all sorts of other weird and interesting implications in chaos theory and philosophy of maths.

You can write a game of life implementation on a rainy sunday afternoon, and be stuck playing with it for weeks.

This guy took it to another level though.