One day some 74,000 years ago, in a swampy valley in the south of India, dawn never came. In the half-light, greyish dust sifted down, blanketing the ground and turning trees to ghosts. Far to the east, a volcano called Toba on the Indonesian island of Sumatra had unleashed one of the greatest eruptions ever known, flinging thousands of cubic kilometres of rock into the atmosphere and spreading a pall of ash across southern Asia.Clive Oppenheimer, a volcanologist at the University of Cambridge, UK, has studied the ash deposits in India's Jurreru Valley to reconstruct the events that followed. Within days, the trees shed their whitened foliage; rains later swept ash into layers several metres thick on the valley floor.
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