In Alaska, the fresh water that feeds the rivers is snowmelt from North America's highest mountains and most active volcanoes. Time and again, they erupt and poison the rivers. Scientists have only just begun to piece together what might have happened nearly 2,000 years ago, when one race of salmon faced the death of their natal river and were forced back to the open ocean on an exceptional adventure. Navigating between the sulphurous waters, bears, sharks and eagles, the fish escaped the Earth's wrath to give birth to descendants that continue their pioneering journey to the heart of an active volcano.
