The South Korean military was informed about the third tunnel by a North Korean defector, Kim Bu-Seong, who fled the communist state on September 5, 1974. He claimed to have measured the tunnel from a large poplar tree in the southern demilitarized zone.
107 PVC cylinder pipes with a diameter of 7.5 cm were then filled with water and installed every 2 meters along the expected invasion route to the South. Three years later, on June 10, 1978, one of the cylinder pipes exploded, sending water shooting into the air. Therefore, five more cylinder pipes were installed nearby, and South Korea was finally able to penetrate the tunnel that North Korea intended to use for the military invasion.
This interceptor tunnel, which took three months to build, was used to locate the invasion route tunnel. If you descend 300 meters, you will come across the North Korean infiltration tunnel.
The third tunnel is arched and has a total length of 1,635 meters. It lies 73 meters below the surface and is 2 meters high and wide. When first discovered, this tunnel extended 1,200 meters north from the military demarcation line and 435 meters south. Today, visitors can explore up to 265 meters of the tunnel. At the end is the concrete blockade of the third tunnel, which was installed to prevent use by the North. The distance from the third blockade to the military demarcation line is 170 meters.
There is evidence that this tunnel was intended for the Southern invasion.
First, the tunnel is tilted approximately 0.003 degrees to the north, which allows water to drain to the north.
Second, the dynamite holes in the wall were intended to explode bedrock to the south.
Although no coal was ever found in this area, traces were left on the rock face to disguise the tunnel as an abandoned mine.