Story highlights Ex-S. Korean general suspects a North Korean tunnel is under a Seoul apartment block

But S. Korean Defense Ministry believes tunnels can't extend beyond 6 miles from the DMZ

A defector says tunnel operations peaked in the 1980s

Gen. Hahn Sung-Chu never believed North Korea could dig a tunnel that reached Seoul -- until now.

Standing inside a basement of an apartment block in the heart of the capital, the former two-star general in the South Korean military says, "This is a kind of invasion, North Korean soldiers working underneath us."

Hahn says residents had complained of underground vibrations, but the subway does not run beneath them.

He says dowsers detected three tunnels, 13 to 16 feet (4 to 5 meters) wide at a depth of up to 39 feet (12 meters). His team drilled two bore holes to lower a camera, but before they could, they detected two underground explosions and their drill holes were blocked. Hahn is certain that North Korean soldiers were working beneath their feet, protecting the tunnel.

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A history of tunnels

Four tunnels from the North have been found in all, although none since 1990. The South Korean Defense Ministry still officially looks for them as it believes there may be 20 in all, but the budget is small and tunnel hunters believe it is merely a token effort. North Korea has said the tunnels were not for invasion, but part of its mining industry.

While the Defense Ministry believes there may be up to several tunnels dug under the Demilitarized Zone, it is convinced that none would reach as far as Seoul. It believes that Pyongyang would not be able to dig more than 6 miles (10 kilometers) from the DMZ -- the heavily fortified border -- because of the Imjin River. Seoul's northwestern boundary is about 25 miles (40 kilometers) from the DMZ.

"To dig tunnels tens of kilometers, it must be angled properly," says Defense Ministry spokesman Kim Min-Seok. "There is a huge amount of groundwater in the Korean Peninsula, so water and soil have to be removed consistently. South Korea and the U.S. have always taken aerial photographs, and we found no evidence of this."

A view from a former North Korean official

But a former intelligence official from North Korea says the notion is not as far-fetched as it might sound. The military defector, who is not using his name as he still has family in the North, said North Koreans "usually pulled out soil and stones during the night so as not to be detected by the U.S. or South Korea. They dig into the ground vertically from 100 to 150 meters (328 to 492 feet) and slope upwards to South Korea so the water drains back to the North."

The man says he had knowledge of the tunnel digging when he lived in the North. Saying that operations peaked in the 1980s, he believes Pyongyang would still protect the multiple tunnels it dug over many decades.

"I was told the tunnels are not directly connected to the streets of Seoul because of the risk of being detected. The tunnels are connected to the sewers linked to the relevant organizations."

The relevant organizations that would be targeted are the U.S. Embassy, the Blue House presidential compound and government buildings, he says.

The former official says that in case of war, infantry troops would flood into the tunnels dressed in American or South Korean uniforms. The tunnels would then be blown up behind them so there would be no retreat.

But in recent years, many suspected tunnels have been proved false. And concerns about Pyongyang have shifted to its nuclear ambitions and long-range ballistic missiles.

Tunnel hunters do not always get much respect in South Korea. But Hahn, and others who have dedicated their time and money to the search, say the government should not ignore the threat they believe exists under their very feet.