Since the erection of the DMZ the Republic of Korea has found about 3 tunnels going underneath the zone apparently dug by North Koreans (the North Koreans deny this claim, insisting that the tunnels were dug by the South Koreans to get into North Korea). They’ve since turned one of the tunnels into a tourist attraction but for whatever reason don’t let you take pictures while you’re inside the tunnel which is why this isn’t the best angle, but if you look closely the little square in the upper corner, you can see into North Korea.