Based on a tip from North Korean defector Kim Bu-seong – who came to South Korea on September 5, 1974 – that North Korea was constructing underground tunnels, drilling works began at the Munsan area from 1975, but no sign of tunnels was detected at that time. During drilling works on June 10, 1978, however, one exploratory hole exploded, so reverse drilling work was executed. On October 17, 1978, the 3rd Tunnel was discovered 4km south of Panmunjeon. This tunnel is 2m wide, 2m high, and 1,635m long, and 30,000 soldiers could cross into Seoul; the location of the tunnel is only 52km from Seoul, so even though the scale is almost the same as the 2nd Tunnel, it is considered more threatening than the 1st and 2nd Tunnels leading to Seoul. It is located a mere 4km southwest away from Imjingak and 3.5km from Tongilchon Village. It can be reached within 45 minutes by car from Seoul.