Geological time spans are divided into two main categories: eons and eras. An EON is the largest division of geological time, lasting hundreds of millions of years. The current EON is the Phanerozoic, which began 541 million years ago. An ERA is a smaller division of geological time, lasting tens of millions of years. There are three eras in the Phanerozoic EON: the Paleozoic, Mesozoic, and Cenozoic. The Paleozoic ERA began 541 million years ago and ended 251 million years ago. The Mesozoic ERA began 251 million years ago and ended 66 million years ago. The Cenozoic ERA began 66 million years ago and continues to the present day.