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In spite of this "most violent gaseous ebullition," as one early explorer put it, the volcano beneath Yellowstone was long thought to be extinct, as Doane believed, or at least in its dying days. Indeed, after federal surveys in the late 19th century, the volcanic nature of Yellowstone received little scientific scrutiny for dec­ades. Then in the late 1950s, a young Harvard graduate student, Francis "Joe" Boyd, became intrigued by the presence of a welded tuff—a thick layer of heated and compacted ash, which he realized was a sign of pyroclastic flows from an explosive, geologically recent eruption.

In 1965 Bob Christiansen found a second distinct welded tuff; the next year he and his colleagues identified a third. Using potassium-argon dating, they determined that the three tuffs were the result of three dis­tinct eruptions. Each created a giant caldera, with the most recent eruption largely burying signs of the previous two.

Then one day in 1973, Bob Smith and a colleague were doing some work on Peale Island, in the South Arm of Yellowstone Lake, when Smith noticed something odd: Some trees along the shoreline were partially submerged and dying. He had worked in the area back in 1956 and was planning to use the same boat dock as on the earlier trip. But the dock was also inundated. What was going on?

Intrigued, Smith set out to resurvey benchmarks that park workers had placed on various roads throughout the park beginning in 1923. His survey revealed that the Hayden Valley, which sits atop the caldera to the north of the lake, had risen by some 30 inches over the inter­vening decades. But the lower end of the lake hadn't risen at all. In effect, the north end of the lake had risen and tipped water down into the southern end. The ground was doming. The volcano was alive.

Smith published his results in 1979, referring in interviews to Yellowstone as "the living, breathing caldera." Then in 1985, heralded by a "swarm" of mostly tiny earthquakes, the terrain subsided again. Smith modified his metaphor: Yellowstone was now the "living, breathing, shaking caldera."

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