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[Stockbild] Grand Prismatic Spring. Aerial photograph of the large geothermal spring in the Yellowstone National Park, Wyoming, USA. The pool is fed with water that is warmed by volcanic activity beneath it. Grand Prismatic Spring is 90 metres (m) wide, 50m deep and the water's temperature can reach over 70 degrees Celsius (C). The bright colours around the rim of the pool are due to minerals that are deposited from evaporation of water. The colour differences within the pool are due to different species of thermophilic bacteria. These bacteria can tolerate temperatures above 55 C, with each species occupying a different temperature range. (KEYSTONE/SCIENCE PHOTO LIBRARY/BERNHARD EDMAIER) === ===