Dead and dying trees on the south side of Mammoth Mountain were first noticed in 1990. Since then, about 170 acres of trees have died on all sides of the volcano, especially near Horseshoe Lake. When the soil was surveyed in 1994 for carbon dioxide gas, exceptionally high concentrations of gas were found in the soil beneath the trees. What caused such high concentrations of carbon dioxide gas? The most likely sources of the carbon dioxide gas include (1) magma that intruded beneath Mammoth Mountain during an earthquake swarm in 1989; and (2) limestone-rich rocks beneath Mammoth Mountain that were heated by the hot magma. For more information, see USGS Fact Sheet Invisible CO₂ Gas Killing Trees fact sheet and additional information about monitoring carbon dioxide gas at Mammoth Mountain.