----------------------------Original message---------------------------- Things may have changed since I last fooled with IR imagery, but my recollection is that it doesn't take a whole lot of insulation to mask temperature differentials. A layer of snow, or a couple of feed oops, feet of earth are going to cover any subsurface warm spots. What you might pick up is cave entrances, assuming that the cave is exhaling, and that the ambient air temperature is cooler than the subsurface air being exhaled-- which is 54 farenheit if memory serves. It also seems to me that not all caves exhale-- there is one in Virginia called Breathing Cave, but beyond that I don't remember any as being particularly breathy. I don't know what kind of money you have to throw around, but aerial photography isn't exactly cheap... You probably need to talk to someone in the IR business-- it's possible USGS Reston, or the nearest ESIC would have a handle on someone who could give you a better answer. charley seavey aka DOCMAPS