in most soils; however, soil concentrations of in the tree-kill areas ranged from 20% to 90%. This overabundance of was found to be the cause of the tree-kills because tree roots need to absorb O2 directly and the high level reduced available O2. Researchers also determined that Mammoth releases about of every day. As of 2003, the concentration of carbon dioxide in soil gas at Mammoth Mountain is being monitored on a continuous, year-round basis at four sites—three at Horseshoe Lake and one near the base of Chair 19 at the ski area. The most likely sources of the are degassing of intruded magma and gas release from limestone-rich metasedimentary rocks that are heated by magmatic intrusions. The remarkable uniformity in chemical and isotopic composition of the and accompanying gases at different locations around Mammoth Mountain indicates that there may actually be a large reservoir of gas deep below the mountain from which gas escapes along faults to the surface. Measurements of helium emissions support the theory that the gases emitted in the tree kill area have the same source as those discharged from Mammoth Mountain Fumarole. There is evidence that the rate of discharge has been declining, with emissions peaking in 1991. Ski patrol fatalities incident In April 2006, three members of the Mammoth Mountain Ski Area ski patrol died while on duty. All three died from suffocation by carbon dioxide when they fell into a fumarole on the slopes of the mountain. The 2005/2006 winter season delivered significant snow depth of . As the ski patrollers assessed the fumarole for skier safety, the perimeter snow collapsed dropping the one patroller into the bottom of the fumarole where the levels are life-threatening within minutes. A second patroller dropped into the fumarole in an effort to rescue the first and then the third patroller followed into rescue where all succumbed fatally to the toxic gas. Ultimately, a team of responders had to enter the fumarole with gas masks to retrieve the bodies. Recreational use Mammoth Mountain is home to the Mammoth Mountain Ski Area, founded by Dave McCoy in 1953. Mammoth is a ski, snowboard, and snowmobile mountain during the winter months. Mammoth is the highest ski resort in California and is notable for the unusually large amount of snowfall it receives compared to other Eastern Sierra peaks—about 400" (1,000 cm) annually and about 300 out of 365 days of