A "Shellback" is a term used in the United States Navy to describe a sailor who has crossed the equator for the first time. This event is often celebrated with a traditional ceremony known as the "Line Crossing Ceremony," where sailors undergo various rites of passage. The ceremony symbolizes the transition from a "pollywog" (a sailor who has not crossed the equator) to a Shellback. The origins of the Shellback tradition date back to ancient maritime practices. During the ceremony, sailors may face playful challenges and receive a certificate to commemorate their crossing. The term is also associated with the King Neptune figure, who presides over the ceremony, adding a sense of fun and camaraderie among the crew.