Abelian Sandpile Model (1987)

Introduced by Bak, Tang, and Wiesenfeld, this is the foundational model of "Self-Organized Criticality" (SOC).
Grains of sand are constantly dropped onto the center of the grid.
If a cell accumulates 4 or more grains, it becomes unstable and "topples", sending 1 grain to each of its 4 neighbors.
This simple spilling mechanic creates a system that naturally evolves toward a critical state where one single grain can trigger an avalanche of any size, creating stunning fractal patterns.