Explosive charge buried just below the surface of the earth, used in military operations against troops and vehicles. It may be fired by the weight of vehicles or troops on it, the passage of time, or remote control. Though improvised land mines (buried artillery shells) were used in World War I, they only became important in warfare during World War II and have been widely used since. Most early mines had metal cases; later models were sometimes made of other materials to prevent magnetic detection. They are typically used to disrupt or prevent the massed attack of tanks or infantry, but in post–World War II conflicts they have also been used to render land useless to enemy civilian populations. A treaty banning land minesnot signed by the U.S., Russia, and Chinawent into effect in 1997. See also submarine mine.