Geothermal mud pool. Hot mud in a pool formed from geothermally heated water. 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 in Orakei Korako, a geothermal valley between Rotorua and Taupo, on New Zealand's North Island.