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Infiltration Tunnel No. 4 was discovered on March 3, 1990. It is located in an area 26km northeast of Yanggu in the eastern front of the DMZ. Located 145m underground, the tunnel is 2m wide and 2,052m long. It was revealed that the tunnel excavation has progressed to 1,052m into the South from the MDL.
The tunnel was discovered as a result of a thorough analysis of defector reports and other intelligence as well as up-to-date detection technology. In May 1989, after the sound of a motor was heard near the tunnel area, the tunnel detection team used up-to-date detection equipment developed by the Korea Scientific Technology Institute for such purpose.
By sending signals into holes, which were drilled at 20m intervals, via antennas and analyzing the signal results, they successfully learned the exact location and size of the tunnel. Subsequently, the South began a reverse tunnel excavation with the presence of reporters, and the reverse tunnel penetrated the tunnel that the North had excavated and exposed Infiltration Tunnel No. 4 to the world.
With Tunnel No. 4, it was shocking to the South to discover objective evidence of the North having excavated infiltration tunnels well into the late 1980s.

Infiltration Tunnel #4

Infiltration Tunnel #4