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Yellowstone sits above the molten core of a volcano. Underground water becomes super-heated and erupts into the air. And it's these geysers that make Yellowstone famous. Strangely, geysers and hot springs can creep around the landscape. The curious shapes of Grotto geyser are actually trees entombed and petrified by minerals dissolved in the water. Hot water scorches and drowns the roots of trees encroaching on the valleys. It can keep the forests in check and the rich pastures open. Those pastures now produce the new grass needed to make rich milk and feed a bison baby boom.