The first sign of an impending cataclysm in the summer of 1831 was an eerie dimming of the Sun, which for days appeared bluish green across the Northern Hemisphere. In the ensuing weeks, foul weather and a long cold snap triggered crop failures and famines in India and Japan. The instigator was long presumed to be a climate-altering plume from a major eruption, but the volcano’s identity had been one of the great unsolved mysteries of volcanology. “It’s like a whodunit,” says Clive Oppenheimer, a volcanologist at the University of Cambridge. At long last, the culprit has been unmasked in...