The first of the tunnels was discovered by a South Korean Army patrol, noticing steam rising from the ground. The initial discovery was met with machine gun fire from North Korean soldiers. Five days later, during a subsequent exploration of this tunnel, U.S. Navy Commander Robert M. Ballinger and ROK Marine Corps Major Kim Hah Chul were killed in the tunnel by a North Korean explosive device. The blast also wounded five Americans and one South Korean from the United Nations Command.