Norris Geyser Basin is a raw, acidic landscape where iron and arsenic abound and thermophiles and extremophiles, microorganisms that live in heat and other extremes, inhabit geysers and hot springs. Many pools are opalescent, or cloudy, and their murky waters are caused by silica in the water. Colloidal Pool is usually a perfect example, although at Norris Geyser Basin changes are common and any perfect example might change at any time.