In April 2010 the eruption of Iceland's Eyjafjallajokull volcano turned much of the northern hemisphere into an ash-strewn no-fly zone, stranding hundreds of thousands of travelers. But Eyjafjallajokull was just the start. Now, an Icelandic volcano 10 times bigger, Katla, has begun to swell and grumble. Two more giants, Hekla and Laki, could erupt without warning. Iceland is a ticking time bomb: When it blows, the consequences will be global. As CGI takes us inside these geological monsters, we meet atmospheric scientists who are working to understand just how devastating an eruption could be -- not just for air travel but for the global food supply and for Earth's climate. Could we be plunged into years of cold and famine? What can we do to prepare for the disaster to come?