Fumarole, sometimes spelled fumerole, is an Italian word for a hole or vent in the Earth’s surface from which fumes, smoke, or vapors issue. Fumaroles are found at the surface of lava ﬂows, in the calderas and craters of active volcanoes, and in areas where hot, intrusive, igneous rock bodies occur. They emit powerful jets of volatile constituents— steam, hydrochloric acid, sulfur dioxide, and ammonium chloride—at high temperatures, sometimes reaching one thousand degrees Celsius. Although usually associated with volcanic landscapes that are dormant or declining, the fumarolic stage may actually precede volcanic eruptions, as was the case with Mount St. Helens in Washington State in 1980. Other areas of fumarolic activity include the Valley of Ten Thousand Smokes near Katmai Crater in Alaska; Yellowstone National Park; and Bumpass Hell on the ﬂanks of Mount Lassen in northeastern California. Fumaroles may stay active for decades or even centuries if they are above a persistent heat source, or they may disappear within weeks to months if they occur atop a fresh volcanic deposit that quickly cools.