The Christian preacher believes the tunnels now extend so far south that one has reached this town, 63 kilometers, or 40 miles, from the border, passing underground just a few hundred meters from the building where he ministers to a congregation of nine families.

“I suspect the North Korean tunnels have spread like a spider web underneath Seoul,” the South Korean capital 50 kilometers from the border, Mr. Kim said. “Imagine hordes of crack North Korean troops streaming out and taking the whole city hostage!

“Some people think I’m crazy, and that saddens me.”

Mr. Kim, 47, is one of a small but dedicated band of South Koreans who have been hunting for North Korean “invasion tunnels” for years, some for decades. Only four tunnels have ever been detected, all between 1974 and 1990 and all near the border. Not one has been found since, despite thousands of drilling operations not only by the South Korean military but also freelance prospectors like Mr. Kim.

Although broadly dismissed as cranks, the private tunnel hunters are not giving up. And the headlines they generate with their recurring claims tap into the source of one of South Koreans’ greatest fears about North Korea: its penchant for taking its war preparations underground, a reaction to the leveling of its military installations by U.S. air power during the 1950-53 Korean War.

A reminder of that subterranean threat came in May, when Brig. Gen. Neil H. Tolley, commander of U.S. Special Operations in South Korea, was quoted as saying at a conference in Florida that North Korea is thought to have constructed thousands of tunnels and other underground military facilities. These include 20 partially subterranean airfields and thousands of underground artillery positions in addition to the four tunnels discovered near the South Korean border. But he cautioned that intelligence about such hidden structures was hard to come by.

“We don’t know how many we don’t know about,” said General Tolley, according to The Diplomat, a Japan-based foreign affairs magazine.

The South Korean Defense Ministry said it remained on the lookout for possible tunnels by drilling along the border and applying advanced sound-detecting and other equipment.

“We don’t ignore even the smallest signs of a possible tunnel because these tunnels could determine the outcome of a war and our country’s survival,” it said in a faxed reply when asked about the tunnel hunters’ accusations that the ministry is not doing enough.

But it also said it was technically impossible for the North to dig tunnels as far south as people like Mr. Kim say.

“Most of those who claim to have found a tunnel are using unscientific methods,” it said. “All their claims have proved false.”

The tunnel hunters include retired military intelligence officers, Christian pastors asserting that God has revealed a tunnel location, even a near-blind Roman Catholic priest who says he has discovered 17 invasion tunnels with a dowsing rod. Often acting on tipoffs from villagers who report hearing mysterious noises under their houses at night, they pull into a village and start digging or drilling. As weeks and then months go by without locating a tunnel, once-curious villagers turn hostile. The authorities arrive with a “It’s you again” scold or order them to fill up the holes and leave.

The tunnel scare began in 1974, when South Korean border patrols saw steam shooting up from the ground. Drilling revealed a tunnel that had reached about one kilometer south of the border. To this day, North Korea denies having dug any such tunnels.

The South Korean military maintains two of the tunnels for tourists to warn younger generations about the danger it says North Korea still poses six decades after the Korean War ended in a truce. The narrator in a video shown to visitors intones: “Listen carefully. You might hear a faint noise from a motor running in the dark underground.”

He continues, “As long as underground provocations continue, there can be no true peace above the ground.”

Few take this exhortation more to heart than the Rev. Lee Jong-chang, 78, a Roman Catholic priest and veteran tunnel hunter. “I am doing this work so that we won’t have another war and nobody will be hurt,” he said.

Father Lee said he was helping villagers find ground water with dowsing skills he had learned from a French missionary when the government asked him and other civilian experts to join the military’s search for tunnels in 1974. He was awarded a presidential medal for helping find the second tunnel, in 1975.

He has since made hundreds of field trips with his dowsing rod, pursuing what he calls a “mission from God.” Some church authorities disapproved of the extracurricular activity and sent him to Ecuador as a missionary from 1987 to 1989. That failed to weaken his resolve.

Now living alone and slowly losing his eyesight to diabetes, Father Lee keeps a patchwork of maps that covers his living room floor. They show what he asserts are the routes of the 17 tunnels he has detected. His hand-drawn maps show tunnels branching out underneath Seoul and stopping beneath primary school playgrounds or approaching the walls of subways, with the last few meters of earth to be broken through, he said, once Pyongyang decides to invade.

“With my rod, I can see underground as vividly as a good doctor reads a patient with his stethoscope,” Father Lee said. “But some people and the military call me superstitious.”

At least some defectors from North Korea share Father Lee’s concern.

“We were always told that tunnels were a key infiltration route,” said Lim Chun-yong, who said he served in a North Korean special forces unit for 16 years before defecting to the South in 1999. Kim Tae-san, a North Korean trade official who defected to the South in 2003, said his friends in the military engineering corps had told him in the 1980s that reaching Seoul’s subway system via tunnels was part of North Korea’s plan to invade the South.

Mr. Kim, the preacher, regrets that South Koreans no longer pay much heed to such talk. To young South Koreans, he says, the tunnels are little more than relics of the cold war.

“It’s like throwing an egg at a rock,” he said of his efforts to raise alarms about invasion tunnels and raise money for new drilling operations. The last private drilling was conducted north of Seoul last year by a man who has been looking for tunnels for 20 years, so far in vain.

Like a gambler on a prolonged losing streak, people hunting for tunnels sometimes borrow recklessly to finance their passion and end up divorced or on the run from creditors. Mr. Kim calls it “tunnel disease” and concedes that he himself has suffered from it.

He succumbed after a tunnel hunter named Chung Ji-yong showed up in his neighborhood in 2002 and began digging. Since then, Mr. Kim has exhausted the money he had saved to build a new church. When he began preaching in Hwaseong, in 2000, his church had 80 members. Now there are 25. Nonetheless, Mr. Kim sounded convinced of his “patriotic duty.”

“My church may be at risk,” he said. “But my country is at much greater risk. The invasion tunnels are North Korea’s last secret weapon.”