Wart treatment. Wart on the sole of a foot after treatment with liquid nitrogen. 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. The use of cold to kill tissue is called cryosurgery. The nitrogen freezes the wart solid. As the wart thaws, a blister forms, lifting it off the skin. Warts are growths on the skin caused by the human papillomavirus (HPV). Photographed during a wart clinic at a general practice doctor's surgery. For the wart during treatment see image M729/017.