University of Chicago associate professor Dorian Abbot chats about his research on the controversial Snowball Earth Hypothesis.  That is, the idea that at least twice in Earth’s geological past, our planet was basically a glacial ball of ice and snow.  Abbot says it likely happened some 2 billion years ago and again some 600 to 800 million years ago.   There's evidence for at least four such snowball events that likely persisted for tens of millions of years.