Media playback is unsupported on your device Media caption Lee Chang-gun on why he and his colleagues keep an eye on the tunnels

To become a tunnel-hunter in South Korea, you need a pump, a generator and a bit of imagination.

Lee Chang-gun and his band of investigators have all three.

They spend their free time searching for North Korean infiltration tunnels here in the mountains near their country's northern frontier.

A motley crew of priests, believers and ex-soldiers, their mission on the day I meet them is to explore a mountain-side hole spotted by a local citizen.

"I'm not jumping to any conclusions," Mr Lee says, as he dons his hardhat and rubber boots.

"But it's definitely a man-made tunnel. There's evidence of explosives, several bore-holes in the walls, and evidence of activity, so we need to find out more."

'Concerned'

Inside the tunnel, the air quickly becomes cold and clammy. Water drips from the ceiling, onto the rocky floor. But the tunnel is large and regular, and has gripped the tunnel-hunters' attentions.

Mr Lee and his crew believe that North Korean agents dug this tunnel as part of a vast secret network stretching under South Korean territory.

They point out what they say is evidence of explosives and man-made holes in the walls.

The military are so passive in responding to these findings, that people have no choice but to come to us Pastor Kim Jin-chol, Tunnel-hunter

Today they are hoping to drain a pool of water from the end of the tunnel to find out where it leads. But as they lug the pump and generator up the slippery path to the entrance, an army vehicle cruises to a stop at the roadside.

A lively debate ensues before the officer moves them on. Not everyone has faith in the tunnel-hunters' theories.

Several of the group are left very annoyed, including Pastor Kim Jin-chol.

"The military says this isn't a North Korean tunnel," he says. "But a local civilian came to us because he was concerned about it. The military are so passive in responding to these findings, that people have no choice but to come to us."

The military deny that they are passive, saying that they devote specialist resources to finding infiltration tunnels.

Four tunnels have already been found here - most in the 1970s - and the army admits there may be many more.

But it has been 20 years since a new find and these days, the tunnels verified as genuine North Korean networks are seen more as tourist attractions than military threats.

Self-belief

Lee Chung-min, professor of international relations at Yonsei University, says the tunnels are much less significant to North Korea's military strategy than they were before, for a number of reasons.

Image caption The army admits that there may be more infiltration tunnels

"One, because the North Koreans have over 900 missiles targeted against the South, so they can hit almost any target in South Korea.

"Two, they have long-range artillery which can reach Seoul in minutes, so the net asset of the tunnels today in 2012 is much smaller than it was in 1975 or even in 1990."

And as the importance of the tunnels has declined, so have the fortunes of the tunnel-hunters themselves.

There was a time, a decade ago, when money was no object and men like Pastor Kim could count on the public's support for major digging projects in their community.

Since then, Pastor Kim says he has spent over $100,000 (£63,700) and lost a large proportion of his congregation into the bargain.

These days, the pews stand largely empty at his regular Sunday service - most of the flock having fled from his weekly tirade against the Northern threat.

Many tunnel-hunters have lost even more, he says: their families and their savings.

But belief in the worth of their unusual hobby has not wavered yet.