In April, 2010 the eruption of Iceland's Eyjafjallajokull volcano turned much of Europe into an ash-strewn no-fly zone, stranding millions of travelers. But was Eyjafjallajokull just the start? Now, an even more threatening Icelandic volcano, 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 could 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.