Wart treatment. General practice doctor using a magnifying glass to examine the circular marks left by warts that were killed with liquid nitrogen. The use of cold to kill tissue is known as cryosurgery. Liquid nitrogen freezes warts solid. As they thaw, blisters form, lifting them off the skin. Warts are growths on the skin caused by the human papillomavirus (HPV). Photographed during a wart clinic at a doctor's surgery.