: Going underground has always been considered an excellent way of protecting vital military installations. The French had their Maginot Line in the 1930s, and the Nazis had their V-2 rocket factories a decade later. But as marvelous as those underground constructions were in their day, they were overtaken long ago by the hidden world created by the Stalinist rulers of North Korea.

North Koreas obsession with tunneling was first noticed during the 1950-1953 Korean War. The Norths first ruler, the late dictator Kim Il-Sung, was awed by the efficiency with which American airpower leveled his country. The resulting mania for digging has led to the creation of the current underground network of at least 8,000 sites, including hideaways for the Norths tiny but elusive nuclear arsenal.

The part of North Koreas underground world that has attracted the most foreign attention has been the tunnels dug by the North Korean Army beneath the DMZ (Demilitarized Zone) separating North Korea from the democratic South. Between 1974 and 1990 four tunnels were detected by American and South Korea forces. These tunnels were up to 3,000 meters in length and were capable of infiltrating up to 30,000 troops an hour, including light-armored vehicles and artillery. One tunnel even had a small plaza in which troops could be called into formation. South Korean sources speculate that up to 20 more tunnels may be lying dormant beneath the DMZ.

North Koreas navy hasnt lacked for shelter, either. The North has protected its sizable fleet of light surface units and submarines inside tunnels leading directly to the sea. These tunnels are 200-900 meters long and 14-22 meters wide, just the right size for a missile boat or a commando-carrying mini-sub.

More worrying to South Koreas defenders than buried naval bases are the 4,000 artillery pieces the North Koreans have deployed behind the DMZ. Including 500 long-range rocket launchers and guns that can pound the South Korean capital of Seoul from 50 kilometers away, Northern artillery is heavily dug in with each gun or launcher protected by its own individual bunker. All artillery bunkers are concealed on the reverse slopes of hills, where theyre safe from direct observation. A gun can be slid out on rails, fired, and then quickly returned to safety behind the armored doors of its underground shelter. Each gun has its own self-contained ammunition store as well.

As intimidating as the DMZ fortifications are, theyre supplied by an equally formidable subsurface industrial complex. Sketchy details gleaned from defectors and other sources have provided images of this subterranean economy. Some 180 war-related factories have been dug into the rugged mountains along North Koreas border with China. Included among their number are 35 ammunition factories, 17 artillery plants, four for armored vehicles, five for communication gear, and most ominous of all: eight plants for making chemical and biological weapons.

The landscape around the Norths notorious Yongbyon nuclear complex is almost certainly riddled with tunnels. During the mid-1990s, officials from the International Atomic Energy Agency (IAEA) had limited access to some of these tunnels. One visitor was shown a tunnel that was several kilometers long that ran underneath a nearby river. The IAEA official reported that the tunnel was big enough to hold machinery for nuclear power and fuel reprocessing plants.

If another Korean war does break out, the North Korean elite has done its best to insure its collective survival. According to  high ranking North Korean  defectors, North Koreas upper crust is dispersed among ten ghettos, all of which are tied together by tunnels, and all the leadership enclaves are in turn connected by secure underground routes to the residence of the North Korean leader, Kim Jong-Il. Kwangs 1998 defection also revealed the existence of a 40-kilometer tunnel system encircling the North Korean capital of Pyongyang. 

By putting so much of their militarized economy underground, the North Koreans have created a gigantic version of the underground fortifications built by the Japanese to defend their island outposts during World War Two. The human cost in taking many of those islands was enormous; the U.S. Marines suffered over 20,000 casualties during the Iwo Jima campaign alone. Invading North Korea would be a far more costly proposition for a potential invader than any Pacific island. 

Still, any system of defense must have its weak points and the same must be true of Kim Jong-Ils shadowy empire of tunnels and caverns. Such weaknesses may be hard to find. The larger North Korean cave complexes are at least 80 meters below ground. The most powerful U.S. smart bomb, the laser guided GBU-25, can only punch down to 30 meters. To counter the shortage of large bunker-buster bombs, American geologists have been poring over old geological survey maps made during the 1910-1945 Japanese occupation of Korea. If fracture points could be discovered in the rock overlaying any potential targets, a well-aimed smart bomb might collapse a bunkers roof.

But simple sloppiness may also increase the vulnerability of Pyongyangs troglodyte empire. Many of the factories buried near the Chinese border rely on small to medium-sized hydroelectric plants for power. Modern surveillance technology would easily spot even a small dam. Also coming under the heading of sloppiness is shoddy construction. Many North Koreans are starved for food and motivation due to the decrepit state of their countrys economy and the suffocating political system that accompanies it. So it shouldnt surprise anybody that many buildings, when finished, are almost unusable. A case in point is the 105-story Ryugyong Hotel in Pyongyang. Completed in 1989, the arrowhead-shaped structure has never been officially opened because of construction defects.

Uninhabitable buildings aside, Kim Jong-Ils nationwide system of tunnels and caverns is designed to turn any American bombing campaign into an enormous shell game-with the U.S. guessing wrong most of the time. And its this high level of uncertainty that, when combined with the small number of nuclear weapons already built by North Korea, helps the nasty regime in Pyongyang stay alive. -- Michael G. Gallagher