From The Mysteries of the Unexplained, pp.274-275: To spend the winter in a cave amidst the snows, at an altitude that varies between 11,000 and 18,000 feet, clad in a thin garment or even naked, and escape freezing, is a somewhat difficult achievement, I observed Alexandra David-Neel in the late 1920s. And yet, she wrote, numbers of Tibetan hermits go safely each year through this ordeal. In her book about the 14 years she spent in Tibet, she gives an account on The Art of Warming Oneself Without Fire up in the Snows. The endurance of these monks, she said, is ascribed to the power which they have acquired to generate tumo. She went on to explain: The word tumo signifies heat, warmth, but is not used in Tibetan language to express ordinary heat or warmth. It is a technical team of mystic terminology.­ It is kept secret by the lamas who teach, and they do not fail to declare that information gathered by hearsay or reading is without any practical result if one has not been personally taught and trained by a master who is himself an adept.­ Sometimes, a kind of examination concludes the training of the tumo students. Upon a frosty winter night, those who think themselves capable of victoriously enduring the test are led to the shore of a river or lake. If all the streams are frozen¡­a hole is made in the ice. A moonlight night, with a hard wind blowing, is chosen.­ The neophytes sit on the ground, cross-legged and naked. Sheets are dipped in the icy water, each man wraps himself in one of them and must dry it on his body. As soon as the sheet has become dry, it is again dipped in the water and placed on the novice's body to be dried as before. The operation goes on in that way until daybreak.­ It is said that some dry as many as forty sheets in one night. One should perhaps make large allowances for exaggeration, or perhaps for the size of the sheets which in some cases may become so small as to be almost symbolic. Yet I have seen some respas dry a number of pieces of cloth the size of a large shawl [Respas wear but a single cotton garment in all seasons at any altitude.] It is difficult for us to get a perfectly correct idea about the extent of the results obtained through tumo training, but some of these feats are genuine. Hermits really do live naked, or wearing one single thin garment during the whole winter in the high regions I have mentioned. I am not the only one who has seen some of them. It has been said that some members of the Mount Everest expedition had an occasional glimpse of one of these naked anchorites.