This paper overviews the occurrence and study of magnetism on nanometer-length scales, that is, at sizes where the natural unit on a ruler would be one-billionth of a meter. Nanomagnetism has fascinating early origins on planet Earth, and we must first go back a couple billion years to get to the beginning of the story. Then we will quickly make some big jumps forward through time, to get back to the present. The take-home message is this: we are very lucky to be here now with the first chance in billions of years to understand nanomagnetism in detail, using the tools provided by physics.