Last Updated Jan 6, 2014 9:00 AM EST
In Iceland, he witnesses firsthand the spectacular eruption of Eyjafjallajokull in 2010 -- an event scientists deemed just a four on a severity scale of eight. Pelley then visits perhaps the most famous active volcano on the planet, Italy's Mount Vesuvius, which sits ominously close to millions of people. But he also learns about the volcano on Earth most likely to produce an eruption that could be an eight -- a globe-changing event -- and it's right here in the U.S. under Yellowstone National Park's "Old Faithful" geyser.
Watch Scott Pelley's full episode below or by clicking here.