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Wart treatment. Wart on the sole of the foot being killed by liquid nitrogen on a cotton bud. The top of the wart was cut off by a scalpel. The use of cold to kill tissue is called cryosurgery. The liquid nitrogen freezes the wart solid. As the wart thaws, a blister forms, lifting it off the skin. A wart on the bottom of the foot is known as a plantar wart or verruca. Typically, it is flattened due to the weight of the body. Warts are growths on the skin caused by the human papillomavirus (HPV). Photographed during a wart clinic at a general practice doctor's surgery. (KEYSTONE/SCIENCE PHOTO LIBRARY/ANTONIA REEVE)