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, algae produce the green colour, with colourful mineral deposits left where the water has evaporated. Photographed in Orakei Korako, a geothermal valley between Rotorua and Taupo, on New Zealand's North Island.