Mammoth Hot Springs Terraces are a hot spot at Yellowtone National Park

At Yellowstone National Park, which covers parts of Wyoming, Montana and Idaho, action stirs below the ground as well as above. The park "sits atop a volcanic caldera, where the water bubbles to the surface at 170 degrees," Cepek wrote in an e-mail. His photo of Mammoth Hot Springs Terraces proves the point. In a park rich with hydrothermal features, these hot springs are a stunning favorite. Here, mineral-rich water rises through the porous ground of limestone, meets air, and forms travertine rock in an ever-altering natural terraced sculpture. Cepek wrote, "This photo shows just how alive our Earth is — always changing, always inspiring."