The Snowball Earth is a geohistorical hypothesis proposing that during one or more of Earth's icehouse climates, the planet's surface was nearly entirely frozen, with little or no liquid water exposed. This global glaciation hypothesis is most commonly associated with the Cryogenian Period, which included two major glacial episodes: the Sturtian (c. 717–660 Ma) and Marinoan (c. 650–635 Ma).