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Geysers. Rainbow in the superheated water and steam being ejected from the Pohutu (right) and Prince of Wales Feathers geysers. Beneath the surface of these geysers are deep natural chambers. The rock terraces in the foreground are made up of sinter, a silicate mineral deposited after the hot, mineral-rich water evaporates. Geysers occur in geothermal rock fissures where water in the chamber is superheated by the surrounding rocks, but is prevented from boiling by hydrostatic pressure. At a critical temperature the superheated water suddenly boils and erupts. These geysers are at Whakarewarewa, New Zealand. (KEYSTONE/SCIENCE PHOTO LIBRARY/BERNHARD EDMAIER)