Nothing is built on stone; all is built in sand. But we must build as if the sand were stone.
—Jorge Luis Borges
Here we begin with a pile of sand and, alas, end with one. We often observe that complex systems, after having developed a beautiful and seemingly robust structure, can collapse in an instant. Consider your body, a collection of billions of cells, each interacting and forming a recognizable and vital you. Yet all of those interactions, all that you are and could be, can cease in only a few minutes if, say, you experience a misplaced shock to your heart. Or consider a civilization such as the Maya in their Classic period, in which a vibrant Mesoamerican culture suddenly falls apart. ...