Located along one of the most strategic routes in the eastern sector, this tunnel was discovered in March 1990 only 26km (16 miles) northeast of Yang-gu. It is 145m (476 ft.) below ground and measures about 2m (6 1/2 ft.) tall and 2m (6 1/2 ft.) wide. Almost identical to tunnels two and three in size and structure, the fourth tunnel burrows 1km (2/3 mile) south of the demarcation line. It was designed to help North Korea infiltrate the Sohwa-Wontong corridor, the main access route to the Yeongdong (Seoul-Gangneung) Expressway. It is also located in Gangwon-do; you can't visit on your own, but it is sometimes included in tours of the DMZ and Panmunjeom.