It doesn't matter how many "secret tunnels" they have because you can bet the South Koreans, Chinese, US and everyone else know where they all are.
Their technology is sheite and they wouldn't last a week if the US did a full scale invasion.
To me, this all goes back to Bush's Axis of Evil speech. They've took out the leadership of Iraq and Libya. That leaves Iran and North Korea. I'm not sure they've made the world any safer although they have spilled a f*ck tonne of blood.
Apparently America and the west killed three million North Koreans in the 1950s in their paranoid vendetta against communism so what's a few million more between enemies.