I would not be surprised if there are undiscovered invasion tunnels South of the DMZ, but I find it hard to believe that the North Koreans could have dug a tunnel underneath the Imjim River that reached all the way to Seoul:
Gen. Hahn Sung-Chu never believed North Korea could dig a tunnel that reached Seoul — until now.
Standing inside a basement of an apartment block in the heart of the capital, the former two-star general in the South Korean military says, “This is a kind of invasion, North Korean soldiers working underneath us.”
Hahn says residents had complained of underground vibrations, but the subway does not run beneath them.
He says dowsers detected three tunnels, 13 to 16 feet (4 to 5 meters) wide at a depth of up to 39 feet (12 meters). His team drilled two bore holes to lower a camera, but before they could, they detected two underground explosions and their drill holes were blocked. Hahn is certain that North Korean soldiers were working beneath their feet, protecting the tunnel. [CNN]
You can read the rest at the link, but I don’t think the North Koreans need to dig a tunnel to Seoul to infiltrate a brigade in case of hostilities. Isn’t that what the KCTU is for?