Mathematician Robert Kaplan traces zero’s journey from Sumerian placeholder mark — first appearing in cuneiform tablets sometime between the sixth and third centuries BC, just a notch pressed into clay to show a column was empty — to an actual number: the earliest surviving zero-as-symbol is carved into an Indian stone tablet dated 876 AD, recording a garden’s measurements as 270 and 50. “If you look at zero you see nothing,” Kaplan writes, “but look through it and you will see the world.”