http://www.sciencemag.org/news/2018/06/ancient-earth-froze-over-geologic-instant

Scientists suspect several ancient snowball Earth episodes when the ice sheets covered the planet.

Earth’s ice is melting at a rapid clip today. But some scientists think that during several ancient episodes, the planet plunged into a deep freeze known as “Snowball Earth,” when ice sheets grew to cover almost the entire planet. However, the number of these episodes, their extent, and just how fast Earth turned into an ice cube have long been a mystery. Now, analysis of a newly discovered rock sequence in Ethiopia supports a Snowball Earth event some 717 million years ago and suggests it took place in mere thousands of years—the geologic equivalent of a cold snap.