Infiltration Tunnel No. 1 was discovered 8km northeast of Gorangpo within the DMZ on November 15, 1974. The tunnel was excavated in an area within 800m of the southern limit line when it was discovered. The location of this tunnel is a mere 65km from Seoul, and it was estimated that over one regiment of armed troops could infiltrate through the tunnel in an hour.

When the tunnel was initially discovered, the UNC proposed to the North to investigate the tunnel jointly, but the North refused the UNC proposal. Moreover, the North made an unreasonable argument that the tunnel had no connection to them whatsoever.

However, evidence clearly showed that the North had in fact built the tunnel and that the tunnel was built from north to south. The UNC inspectors discovered in the tunnel the markings that kept track of the tunnel`s southward progress, and they also found a quantity of Soviet dynamite as well as North Korean telephones in the tunnel.

While inspecting the tunnel on November 15 1974, the US and South Korean side suffered some casualties. Two officers died and six others were wounded as a result of traps deliberately installed by the North.