Concretions are remarkable geological curiosities. They are spherical carbonate formations composed of mineral cement. Concretions are found near and far, from Western Kazakhstan to beaches in California. Usually the size of cannonballs, they form from water eroding a piece out of sedimentary rock. They almost look man-made.

Geologists reassessed the abundance of fossils in Earth’s uppermost rock layers this year, and they believe they found a remarkable sixth global extinction event.¹ Their discovery has two significant implications.