Chalk under a microscope.

“Chalk is composed of extremely small white globules. They look, up close, like snowballs made from brittle paper plates. Those plates, it turns out, are part of ancient skeletons that once belonged to roundish little critters that lived and floated in the sea, captured a little sunshine and carbon, then died and sank to the bottom… Scientists call these ancient plates ‘coccoliths.’”

(via scinerds)