In 1974, the South Koreans discovered a tunnel under the DMZ extending into South Korean territory.  Upon discovery of it, they found evidence that it was dug by North Koreans.  Since then, 3 more tunnels have been found.  A North Korean defector claimed that North Korea had dug as many as 15 tunnels for a surprise invasion.  The tunnels are all gradually slowed up towards the south so any water would flow northward and hence better hiding the tunnels.  The North Koreans denied digging any of the tunnels.  To this day, it is believed North Koreans are still digging and South Koreans are still checking for tunnels.  So we took a tour of one of the tunnels.  Upon discovery of the tunnel, the South Koreans went into it to see where it went and part way through, the North Koreans tried to collapse the tunnel, so part of the tunnel is sealed off.