Geothermal algae. Algae growing in the hot water of a geothermal pool. Geothermal pools form in areas where volcanic activity causes hot water to rise up from beneath the Earth's surface. The hot water is often brightly coloured, due to minerals dissolved in it. Here, the algae produce the green colour. The white areas are calcium deposits. Photographed in Orakei Korako, a geothermal valley between Rotorua and Taupo, on New Zealand's North Island.