In 1951, about one year into the Korean War, American troops positioned near Chorwon, roughly 60 miles north of Seoul. Soldiers, preparing to bombard a nearby village, noticed something strange in the hills—something Private First Class Francis P. Wall would later describe as objects or lights resembling “jack-o’-lanterns” moving across the mountain. One of the objects even approached the battlefield, forcing the Americans to engage. According to Wall’s account, artillery couldn’t damage the...