Plantar warts look like calluses on the ball of the foot or on the heel. They may appear to have small pinholes or tiny black spots in the center. The warts are usually painful and may grow singly or in clusters. Plantar warts are caused by a virus that infects the outer layer of skin on the soles of the feet. If you are not sure if you have a plantar wart or a callus, let your doctor decide. They have several ways to remove them.