The eruption of Iceland's Eyjafjallajökull volcano in 2010 turned much of the northern hemisphere into an ash-strewn no-fly zone. But Eyjafjallajökull was just the start. Katla, an Icelandic volcano 10 times bigger, 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. Meet scientists trying to understand those consequences -- for air travel and for the global food supply and Earth's climate. Could we be plunged into years of cold and famine? What can we do to prepare for the coming disaster?