The first tunnel was discovered in November 1974, in the Western Sector of the DMZ near Korangp'o. With an estimated total length of 3.5 kilometers, it extends one kilometer south of the Military Demarcation Line that divides the DMZ down the middle along its entire stretch. The walls and ceiling of the tunnel, 1.2 meters high and 0.9 meters wide, are reinforced with concrete slabs.
Buried 0.45 meters below ground, it is capable of moving a regiment an hour. Equipped with a narrow-gauge railway, the tunnel was lighted with lamps connected to 220-volt power lines. Rail cars and drainage devices were also found. This tunnel is located only 65 kilometers north of Seoul.