From a Corps of Engineers release (July 2021):
“It was hard to locate the exact point since the point looking from the north was different from the point looking from the south. But by chance, on June 10, 1978, an explosion was detected in an old abandoned borehole, from what was apparently a faulty dynamite charge planted years earlier. Immediately, the FED drilling team carried out reverse tunnel excavation work in the surrounding area, and on October 17, the 3rd tunnel was discovered 15 meters ahead of the tunnel end where the team was previously working.”