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^BBoiling mud pool.^b View of a boiling mud pool at a geothermal spring. The mud has been heated by subterranean hot rocks. Mud pools like this are generally found in areas where the Earth's crust is thin or faulted, allowing the Earth's internal heat to escape. This internal heat comes from the decay of radioactive elements, chemical changes occurring under great pressure, and geological movements. Such areas are also commonly associated with volcanism. Photographed at Dolina Geyzerov on the Kamchatka peninsula, Russia. (KEYSTONE/SCIENCE PHOTO LIBRARY/BERNHARD EDMAIER/SCIENCE PHOTO LIBRARY)