This is a common wart, commonly known as boil, which usually grows on the skin surface. It wasn't big at first, but it got bigger and bigger. This disease is caused by hpv virus, which easily leads to acne on the skin. I've planted it before. It used to be only the size of a mung bean, so I couldn't help it. Then it got out of hand, and it grew four or five, and it kept growing. It was disgusting.