I'm a bit surprised to hear that the CO2 is slowing down - The last time I talked to some rangers up there they told me it was getting worse (maybe about 12-18 months now) - they had done some more closures.

The Mammoth web site says this for the current status (updated Dec 10, 2002)
CO2 trend: The diffuse carbon dioxide gas flux in the Horseshoe Lake tree-kill area has shown little change from the relatively high levels of 50 to 150 tons per day sustained for the past several years.

Hmmmm....
Canie

• Carbon Dioxide and Helium Discharge from Mammoth Mountain