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Grand Prismatic Spring. Aerial view of Grand Prismatic Spring, a large hot spring in Yellowstone National Park, Wyoming, USA. The pool is warmed by volcanic activity beneath it. The bright colours come from minerals deposited around the pool, and from colonies of heat-loving bacteria and algae. Yellowstone National Park, the world's oldest national park, contains thousands of geysers and hot pools. (KEYSTONE/SCIENCE PHOTO LIBRARY/BERNHARD EDMAIER)