Did who talk about tunnels?
I know the North Koreans had plans to build many tunnels under the DMZ to access South Korea. They failed on every one.
Your tour guide.
This is a couple of years old ...
Four tunnels from the North have been found in all, although none since 1990. The South Korean Defense Ministry still officially looks for them as it believes there may be 20 in all, but the budget is small and tunnel hunters believe it is merely a token effort.
http://www.cnn.com/2014/10/02/world/asia/north-korea-dmz-tunnels/