The eruption of Iceland's Eyjafjallajökull volcano in 2010 turned much of the northern hemisphere into an ash-strewn no-fly zone. Katla, an Icelandic volcano 10 times bigger, has begun to swell and grumble, and Hekla and Laki could erupt without warning. Iceland is a ticking time bomb. When it blows, the consequences will be global. What can we do to prepare for the coming disaster?