WTF is this?
Around Lake Tahoe there are some similar things that are used for avalanche control. Search for GazEx. Basically it sets off a little explosion of propane and oxygen.
Thanks! I never would have guessed that they had installed (more or less) rocket engines on mountains to clear avalanches. Given the sign I saw on the way back, it seems like Alta is using just about everything.
EDIT: Not everything, as I learned here.