----------------------------Original message---------------------------- Thanks for your reply, Charley. I suspected that IR film might not be sensitive enough to pick up subsurface heat sources if the temperature differential was too low. I guess we're going to be doing a lot of ridge walking this winter. Many cave do "exhale", an effect which is particularly noticeable in winter because it is similar to steam rising from the ground. Breathing Cave, which you mentioned in your message exhibits an "in-out" air circulation cycle which reverses several times daily (a neat cave, BTW). Frank