As Mammoth mountain is actually a dormant volcano, certain areas have pockets of Co2 trapped beneath the snow, Co2 can cause you to black out, and stop the engines of snowmobiles.- This doesn't affect the normal skiing runs - but be careful if you go cross country. The resort is well researched and monitored though so don't let this put you off!
Mammoth Lakes looks like the prettiest little place ever, but there is to some extent, a quite major danger here (not likely to occur while most visitors are here). The town of Mammoth Lakes sits on the enormous Long Valley caldera (a geological feature somewhat like Yellowstone), which could possibly erupt in the near future. In the 1980's, a series of quakes here made scientists worry that something might blow- but nothing did. A road was constructed as a secondary evacuation route from Mammoth, but since the eruption never came, the road was left unused, and was eventually renamed "Mammoth Lakes Scenic Loop". Geographical features around Mammoth attest to the volcanic history of the area: Inyo Craters, Obsidian Dome, Hot Creek, Devils Postpile. Occasionally, CO2 will seep up from the ground in some areas and kill trees, etc, making areas toxic and unsafe; this is very noticeable around Horseshoe Lake, and on parts of Mammoth Mountain itself.