Found in the DMZ in the military demarcation line, this tunnel was recognized by Korean guards listening to the sound of explosions under the ground in the midst of guard work and drilling through modern equipment. After strenuous excavation on March 19, 1975, the second tunnel for a sudden raid of North Korea was found in the area of the Korean army. The spot of the tunnel is composed of a firm granitic layer, and the total length is 3.5km in the 50m-160m underground area.