North Korea - The DMZ

And you thought Mexicans had it tough. The 38th parallel dividing North and South Korea is perhaps the most heavily fortified border in the world. On the southern side, some 37,500 U.S. troops lie in waiting for Kim Jong Il's horde to come crashing though the gates-in this case, the side of a mountain. For nearly 50 years, the North Korean army has been digging tunnels through the granite mountains on its side of the border into South Korean territory, ending them with only a few meters of rock and soil between the North's stockpiled invading force-artillery equipment, tanks, helicopters, warplanes and troops-and South Korean "sunshine." War is little more than a tap of an ice pick away.

http://www.fas.org/nuke/guide/dprk/index.html