IN THE 1960s, while studying the volcanic history of Yellowstone National Park, BobChristiansen of the United States Geological Survey became puzzled about something that,oddly, had not troubled anyone before: he couldn’t find the park’s volcano. It had been knownfor a long time that Yellowstone was volcanic in nature—that’s what accounted for all itsgeysers and other steamy features—and the one thing about volcanoes is that they aregenerally pretty conspicuous. But Christiansen couldn’t find the Yellowstone volcanoanywhere. In particular what he couldn’t find was a structure known as a caldera.

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