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ABOUT THE COVER: False-color, Landsat image of the Indonesian islands of Sumatra (center) and Nias (lower left). Lake Toba, in the center of the image, partially fills the depression (or "caldera") that was created on Sumatra 74,000 years ago during one of Earth's largest and most recent supereruptions. The outer margins of the lake roughly outline the region that collapsed when over 2500 km³ of magma were released explosively from a subsurface reservoir. Samosir Island, in the center of the lake, formed when the floor of the caldera was later pushed upward-presumably by rising magma. Surrounded by steep walls up to 1000 m high, the lake is 500 m deep and roughly 100 km long by 30 km wide. IMAGE COURTESY OF NASA

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