HWASEONG, South Korea — “Some people think I’m crazy,” says Kim Jin-cheul, a Christian preacher who is convinced that North Korean soldiers are digging tunnels that extend under the capital, Seoul, 30 miles from the border, and have reached this town, 10 miles farther south, where he ministers to a congregation of nine families.

“Imagine hordes of crack North Korean troops streaming out and taking the whole city hostage!”

Imagine. 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 conducted not only by the South Korean military but also freelance prospectors like Mr. Kim.

Although broadly dismissed as cranks, the private tunnel hunters 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 United States 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 United States 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. But officials, while stating that they cannot be sure where North Koreans have dug, say that they believe that the tunneling has been limited to the North.