What about the vapor? Did you check the percentage of the humidity in the cave? If there is 12°C cold and the asics are warm the moisture precipitates on the machines. So the rust will ruin the farm. What kind of solution do you have for this problem?
Bitcoins do not rust. 
On a serious not though they are probably having fans set up to blow the machines.....not in the sexual or literal sense but to keep any moisture off of them, running fans (not used for cooling) is very cheap.
I think I saw a post from them saying they indeed have fans to evacuate the hot air/replace it with the cold air from around the cave.