Despite its name, the demilitarized zone that separates North and South Korea, is the most heavily militarized border in earth. An armistice in 1953 might have stopped fighting in the Korean War, it didn`t eased tensions between the two countries divided by the DMZ.
South Korea, a U.S. ally, is a republic with the world`s 12 largest economy. North Korea, a communist state has a giant military, but a chronically struggling economy. Many South Koreans fear that aging underground tunnels could give North Korea`s military a secret path south.
UNIDENTIFIED FEMALE: Seen as a tourist attraction rather than a military threat these days, an infiltration tunnel dug from North Korea to South Korea, passing under the most heavily fortified border on earth.
Three tunnels were found in the 1970s, one in 1990, nothing since. But the Defense Ministry admits there maybe be 20 in all.
Major General Han Sun-chu (ph) believes he`s discovered a new one under sole apartment block. A former two-star general, now a tunnel hunter, he says residents complained of underground vibrations back in March, (INAUDIBLE) then detected three tunnels up to 12 meters under the basement.
His team drilled down to lower a camera. But before they could, Han says the recorder picked up two underground explosions. Their drill holes were then blocked.
The work he is certain of North Korean soldiers protecting the tunnel. This North Korean defector was a senior intelligence official within the military. He hides his identity, as he still has family in the north.

He says Pyongyang`s tunnel-digging lasted decades, before scaling down in the late `90s. But he believes, existing tunnels would still be protected.
South Korea`s Defense Ministry says a tunnel would not reach sole 40 kilometers or 25 miles from the border, North Korea has said in the past the tunnels are not for invasion, but part of their mining industry.