Underground movement

BENEATH THE DEMILITARIZED ZONE, Korea -- In the tunnel's dank subterranean air, a nightmare scenario of war begins with the steady thud of boots on granite.

South Koreans once feared that thousands of North Korean soldiers would surge through the dark, hundreds of feet below the ground, then burst to the surface far into South Korean territory.

Infiltration Tunnel No. 3, one of at least four tunnels that North Korea burrowed under the world's most heavily fortified border, is both a Cold War relic and a warning of the terror a real war could bring.

Tunnel No. 3, discovered a quarter-century ago and now sealed off, is evidence of the extent North Korea went to in planning for war. The South Korean military estimates the six-foot-wide tunnel could have handled 10,000 North Korean soldiers every hour.

"There are not many options for North Korea on the ground or in the air," said Kim Chang-su, senior research fellow at the Korea Institute for Defense Analyses in Seoul. "But under the surface of the earth, we just cannot stop them."

Building underground

The North Koreans are clearly masters of subterranean construction, in part because of their memory of devastating U.S. air power during the Korean War. They have built military facilities underground, and the Pyongyang subway is hundreds of feet below the surface.

The North Koreans built the tunnel by digging and dynamite-blasting more than a mile, slipping beneath the Military Demarcation Line. The passage burrows 1,435 feet into South Korea, and its average depth is 240 feet.

Pyongyang has always denied it built the tunnel, accusing the United Nations Command that patrols the Demilitarized Zone of a hoax.

Down below, the air is humid. An orange pipe along the ground brings oxygen from the surface, and the South Koreans have dabbed yellow paint on digging marks along the wall that they say reveal the drilling went from north to south.

The tunnel was no rush job. Its builders carefully covered the walls of the passageway with charcoal, presumably to make it look like a coal mine. The tunnel was built at a slight incline so water would not flow south, where its appearance might raise suspicions.

Defector's tip

The plan, according to the South Korean military, was to bore underneath the Imjin River just south of the border, allowing invaders to circumvent front-line defenses on the 25-mile march to Seoul.

That plan was foiled in 1978 when South Koreans, working off a tip from a North Korean defector, bored holes deep into the ground in search of the passage, and filled them with water. When the diggers below set off a blast of dynamite, water gushed out of the hole, betraying their presence.

Realizing they had been found out, the North Koreans apparently withdrew.

Today, the tunnel is still considered a high-security area. Taking photographs inside is forbidden, and the South Koreans have sealed off the passageway heading north with three cement barriers about 100 yards from the border above. They monitor it with surveillance cameras and a poison-gas detector

Some suspect that North Korea built many more tunnels, and others claim Seoul is covering up its knowledge of more passageways.

The first tunnel, with two miles of rail lines, was discovered in 1974. A second tunnel was found the following year after the sound of drilling was heard on the surface. After Tunnel No. 3, a fourth tunnel was found in 1990.

The South Koreans are always searching for more.

"Operations to find possible infiltration tunnels by North Korea are ongoing, and they will continue," said a Defense Ministry official who spoke on condition of anonymity.