The Punchbowl is a large topographic basin in the rugged T'aebaek Mountains (T'aebaeksanmaek) of east-central Korea. Its steep northern rim is part of the Korean Demilitarized Zone (DMZ). This terrain was the target of various infiltration attempts in the postwar period and its potential to host clandestine tunnels was indicated by Allied intelligence studies of the early 1970s. In December 1989, at a depth of 145 m in intrusive granodiorite, a coherent cross-borehole electromagnetic anomaly with a signature indicative of an air-filled cavity was recorded by tunnel-search teams using continuous wave and pulsed ground-probing borehole-radar systems. Natural air-filled cavities at this depth were precluded by geological considerations and ground-water elevations. Target-evaluation drilling followed and, on December 24 at 0130 hours, a drill bit penetrated “Tunnel-4.” A borehole television camera provided clear images of artifacts of human tunnel-construction activity. Two and a half months later, Korean engineers, using a Wirth 3-m tunnel-boring machine (TBM), gained access to the tunnel and confirmed its nature and purpose.

Mapping and surveying of the tunnel revealed that (1) the tunnel was constructed by drill and blast methods in Mesozoic intrusive granodiorite, similar to that hosting previously identified DMZ Tunnel-2 and Tunnel-3, and in Precambrian gneiss; (2) the tunnel slopes up to the south at an average grade of 2.3%, ranging from a low of 0.91% to a high of 3.5%; (3) the average cross-sectional dimensions of the tunnel, 1.6 m × 1.6 m, are somewhat smaller than other hardrock tunnels discovered elsewhere in the DMZ; and (4) standard rock-mass rating schemes do not accurately predict tunnel-construction difficulty for hardrock tunnels smaller than 2 m × 2 m in cross section.