Millions of people around the world live in the shadow of active volcanoes. Under constant threat of massive volcanic eruptions, their homes and their lives are daily at risk from these sleeping giants. From Japan's Mount Fuji to the "Sleeping Giant" submerged beneath Naples to the Yellowstone "Supervolcano" in the United States, NOVA travels with scientists from around the world who are at work on these sites, attempting to discover how likely these volcanoes are to erupt, when it might happen, and exactly how deadly they could prove to be.