mofette, fumarole, or gaseous volcanic vent, that has a temperature well below the boiling point of water, though above the temperature of the surrounding air, and that is generally rich in carbon dioxide and perhaps methane and other hydrocarbons. When the winds are right, the issuing gases may drift and settle into nearby hollows or small valleys and cause the asphyxiation of animals and birds wandering in the areas. Such potentially deadly hollows have been noted in the Absaroka Range near Yellowstone National Park in Wyoming, U.S., and at the base of Iceland’s Hekla volcano.