Visitors to the de-militarised zone separating the two Koreas can glimpse a bizarre tourist attraction - tunnels built by North Korea as part of Cold War invasion plans.
Four tunnels have been discovered so far, the latest in 1990, and, as Caroline Gluck reports, some South Koreans believe the North may still be digging.
I donned a hard hat and joined a group of visitors on a tour deep beneath the world's last Cold War frontier.
Some people believe this is the site of a new North Korean tunnel
We were there to look inside an invasion tunnel built by North Korea. It was designed for a surprise attack on the south's capital, Seoul, and built wide enough for 10,000 troops to have passed through in just one hour.
Lieutenant Choi Song-han, our guide, explained the tunnel's significance.
"If this tunnel was not discovered, North Korea can get access beyond the north west boundary of Seoul, they can hit our back at the same time," he said.
It is dark and wet underground. But that has not deterred around 1,000 people from coming to visit each day.
Some are tourists, like Chris Rogers, from the United States.
Why haven't we seen any invasion? What are they waiting for? They are waiting for the right time
Yoon Yeo-kil, Invasion Tunnel Hunters group
"There aren't a lot of places in the world like this, so it makes you think a little bit. People were actually down there, making a tunnel to burrow underneath another country to sneak up and attack them, that's crazy, you know," he said.
The tunnel drilled into the South is one of four discovered by officials.
But about 100 kilometres away, a group calling themselves the Invasion Tunnel Hunters claim to have evidence that at least 20 others have been dug.
Under the gaze of the military at a base south of Seoul, the tunnel hunters have excavated a huge site where they believe a North Korean invasion route is buried. They have worked here for the past six months, to find solid evidence in the face of official scepticism.
One of the group, Yoon Yeo-kil, shows off what they have found - broken steel cables and metal digging tools discovered underground. A tape recording from the site suggests the sounds of underground hammering and muffled voices, he said.
Mr Yoon is convinced North Korea is a real threat.
Mr Yoon is convinced of the North's threat
"I don't think they change at all. They are still very aggressive, and they have the strong intention to occupy South Korea, under their control.
"We are 60 kilometres from the border, a tunnel has got this far. Why haven't we seen any invasion. What are they waiting for? They are waiting for the right time," he said.
Several hours' journey away, I met one man who knows more than most about North Korea's hidden tunnels.
Cha Do-soo, a defector from the North Korean army, now lives in a block of flats, 14 floors above ground. But he has spent much of his life digging underground.
"In the 1990s we were preparing to unify the peninsula and tunnels were part of our everyday life. When I was in the military I heard rumours all the time of commandos freely going back and forth to South Korea through tunnels," he said.
That is the fear of the tunnel hunters. They have been searching for the underground invasion routes for the past 14 years - but have not uncovered one yet.
Now they have run out of money and are deep in debt. Still, they say, they will not give up - and are determined to prove their critics wrong.