"It was eerie," said David M. Finkelstein, director of Project Asia at the Alexandria, Va., nonprofit research center CNA Corp. and a former Army officer posted at the Panmunjom truce village in the early 1980s. "I was absolutely amazed at how wide and high the tunnel I visited was."
There haven't been any major discoveries in recent years, leading some to conclude the North has focused its tunnel-building exclusively inward.
Others aren't so sure, however, and accuse the South of hiding such finds in keeping with its accommodating "sunshine policy" toward its mercurial northern neighbor.
The fear that Northern soldiers might one day pop out of a Seoul or Incheon sewer has spurred on a small group of avid South Korean tunnel hunters who comb the country, poking microphones, cameras and lasers into the ground in search of the North's mole holes.
"Kim Jong Il's regime can only collapse, but even a rat will charge if driven into a corner," said Kim Han-sik, a pastor who heads a prominent tunnel-hunting group. "We don't know when the country could be taken by the North Koreans."
Back in the Beijing tunnel network, guides sporting Chairman Mao pins and camouflage outfits over their designer shoes steer visitors through a small part of the maze deemed safe to navigate.
In a sign of just how far China has drifted from the days of invasion obsession, however, a large underground military planning hall has been turned into a well-stocked silk and souvenir market.
"Business is good," Shao said. "We give you very good price for the quality."