Geothermal mud pool. Eruption of volcanic gases from a mud pool in a geothermal area. Geothermal pools form in areas where volcanic activity causes hot water to rise up from beneath the Earth's surface. Depending on the type of rocks and soil, the water will either form a pool of clear water, or a mud pool. Photographed at the Wai-O-Tapu (Sacred Waters) Thermal Wonderland Park, near Rotorua, on the North Island, in New Zealand.