Overview: Did a cataclysmic event plunge humankind into the period known as the early Dark Ages? Scientists now believe the early Dark Ages may have been triggered by a natural event that occurred around 535 A.D. Science writer David Keys is convinced that the cause was a phenomenon of cataclysmic proportions. At the center of a complex chain of events seems to be "a loud bang" -- a volcanic explosion equal to "two thousand million Hiroshima size bombs." The subsequent environmental calamity, Keys believes, affected human civilization from Mongolia to Constantinople, precipitating plague, famine, death, great migration, the fall of the great Mexican city of Teotihuacan, the Anglo-Saxon victory over the Celts and perhaps even the rise of Islam. (UK / PBS) Released to video (VHS) as a single episode. Run time 1:50.