Like so many worker ants, the North Korean soldiers spent their days underground in a vast labyrinth of tunnels.
Their daily commute involved walking down four steep flights of stairs, then passing through a corridor that went nearly 800 yards into a mountain. They carried tightly sealed cartons, believed to contain raw materials for North Korea's secret weapons programs. Some days, especially if they were being punished, they were assigned simply to dig more tunnels.
K., a North Korean in his 30s, was recruited at age 17 into an elite military unit working for the agency responsible for weapons production. He took an oath to work underground for the rest of his career and was assigned to a cave in remote Musan County in North Hamgyong province, about 15 miles from the Chinese border.
"This is how we hide from our enemies. Everything in North Korea is underground," said K., who described the cave on condition that he be quoted using only his first initial and that certain identifying details be kept vague.
North Korea is riddled with caves like the one in which K. worked. Under its paranoid regime, virtually everything of military significance is manufactured underground, whether it's buttons for soldiers' uniforms or enriched uranium for nuclear weapons. A South Korean intelligence source estimates North Korea has several hundred large underground factories and more than 10,000 smaller facilities. Joseph S. Bermudez Jr., the author of three books on the North Korean military, puts the total number between 11,000 and 14,000.
Whatever the count, there are enough underground facilities that a popular joke has the country eventually collapsing not from the failure of its communist economic system but from so much burrowing in the dirt.
"The place is like Swiss cheese, there are so many holes," said John Pike, a defense analyst with GlobalSecurity.org, a consulting firm in Alexandria, Va.
North Korea's relentless tunneling has had a profound effect on the U.S. debate over how to respond to its drive to build nuclear weapons. It makes the option of preemptive military action far less viable, because so much of the nuclear program is underground and out of reach.
Even if the Pentagon were to develop nuclear "bunker-busters" -- relatively small bombs that penetrate the surface before exploding -- the United States would be hard-pressed to use them successfully without knowing which of the thousands of bunkers scattered throughout the country were the correct targets.
The North Koreans began tunneling after the 1950-53 Korean War, when U.S. bombing destroyed most of their industrial base and infrastructure. North Korea's founder, the late Kim Il Sung, is believed to have been so awed by American air power that he directed key industrial facilities to be built underground.
"The entire nation must be made into a fortress," Kim wrote in 1963. "We must dig ourselves into the ground to protect ourselves." North Korea's mountainous topography, inhospitable for agriculture and transportation, proved to be singularly well suited to the nation becoming what Bermudez calls the "most heavily fortified country in the world."
The nation of 22 million people created the fifth-largest army in the world, with the mountains providing natural cover for its military infrastructure.
"We would dig horizontally into the mountains rather than going straight down because we didn't have good technology for waterproofing and we didn't want to run into the water table," said Lim Young Sun, a North Korean defector who worked from 1980 to 1993 in a construction bureau assigned to build underground facilities.
Lim said the North Koreans used mostly Japanese tunneling techniques, although more modern equipment was later imported from Europe.
In the countryside, small entryways can be seen dug into the sides of many hills, covered with slabs of concrete. "As you travel around and look around, you see that what looked like a regular hill is actually a bunker. It takes a while for your eyes to adjust and to make the mental shift, but after a while, you realize that all of North Korea is an underground facility," said Peter Hayes, who has traveled several times to North Korea and is executive director of the Nautilus Institute, a Berkeley think tank.
Above the demilitarized zone that divides the Korean peninsula, the North Koreans have put an estimated 13,000 heavy artillery pieces into mountain bunkers. The doors face north -- the artillery is positioned to quickly slide in and out on rails -- so that South Korean and U.S. troops stationed south of the DMZ can't reach them.
North Korean tunneling activity hasn't stopped at the border. Over the years, four infiltration channels have been discovered in South Korean territory. Based on defector testimony, South Korean investigators believe that as many as 20 more may still be hidden beneath the earth.
The subway system in Pyongyang doubles as a bomb shelter. Some stations in the capital are believed to be as deep as 100 yards underground, with secret tunnels designed to transport the leadership in an emergency.
One tunnel is said to lead north to Pyongyang's international airport, which is believed to have a runway that is largely underground so that an aircraft would not be exposed to hostile fire until the moment its wheels leave the ground. Predictably, official maps of Pyongyang do not show the location of subway stations, and with the exception of two showcase stations, the system is off-limits to foreigners.
Because so much happens below the surface, the North Koreans are able to conceal their military infrastructure from the prying eyes of surveillance satellites and aerial reconnaissance. But people and vehicles going in and out of the tunnels can be surveyed, as can utility lines.
When a new facility is built, its size can be estimated by the debris, or tailings, excavated in the process. Exactly what happens inside those installations remains shrouded in mystery, however, making North Korea a far bigger challenge from an intelligence standpoint than Iraq.
"North Korea is the longest-running failure in the history of American intelligence," said Donald P. Gregg, who served as both CIA station chief and U.S. ambassador in Seoul. He cited the difficulty of recruiting reliable spies inside the country and the many underground facilities. "It is simply a much harder place to gather intelligence than Iraq or Iran ever was."
For example, confusion reigns on the question of whether North Korea has, as it claims, extracted plutonium from 8,000 nuclear-reactor fuel rods and, if so, where the reprocessing has taken place -- the critical step in producing the fissile material that is the heart of a nuclear bomb.
Until recently, it was assumed that the only possible location was at Yongbyon, a sprawling compound 55 miles north of Pyongyang that has a building six stories high and the length of two football fields designed expressly for plutonium extraction.
United Nations weapons inspectors were expelled from the premises at the end of last year. Now it appears that this imposing building, clearly visible in satellite imagery, might be a decoy. The actual reprocessing could have taken place at undiscovered locations underground.
"I'd be prepared to suggest that Yongbyon is a decoy," said Pike of GlobalSecurity.org. "They don't really need a building the size of several football fields. They could use several buildings the size of tennis courts. Could they hide those? Absolutely. The North Koreans are very annoying that way. What they are trying to hide is small, and they have plenty of places to hide it."
There's even more confusion about the North's production of highly enriched uranium, also used in nuclear warheads. The process requires centrifuges, small devices that are even easier to conceal than the technology used in processing plutonium. The North Koreans asserted in October 2002 that they had a secret uranium program, but U.S. and South Korean intelligence have been unable to determine where.
"Uranium does not give you the same punch for the pound as plutonium, but it has the advantage that it is almost impossible to detect, and you can easily hide it underground," said one U.S. military intelligence official, who requested anonymity.
There is a short list of suspected sites for the uranium program, among them underground facilities at Yongjori, about 12 miles from the Chinese border in Yanggang province, and in Chagang province, also in the north.
The facility in Chagang, known as Hagap, has been known to U.S. intelligence since 1996 and suspected of being variously a reprocessing facility, a high-explosives test site or even an underground nuclear reactor. But Daniel Pinkston, a North Korea military analyst for the Center for Nonproliferation Studies in Monterey, Calif., says more recent information suggests that it is merely a vast underground archive for the ruling Korean Workers' Party.
The North Koreans help maintain the extreme secrecy of the underground facilities by keeping personnel virtually locked inside. This is particularly true for facilities that are used for weapons of mass destruction.
"Once you go in, you don't go out," said K., the North Korean who worked at the Musan County facility until, through a combination of bribery, guile and family connections, he escaped in 1996. "I volunteered for this, but then I came to realize that it was like a big prison and we were slaves."
K. was assigned to work for the 2nd Economic Committee of the National Defense Commission, which despite its innocuously bureaucratic name is the organization believed to be responsible for producing weapons of mass destruction. He said he was selected for the elite unit because his father also was in the military and his family was considered loyal to the regime.
At the time he was sworn in, he took an oath promising to work there until age 60. In nine years, he left only once -- handing over a bribe so he could visit his family at home. Others could see relatives only at a reception area outside the facility, where visits were supervised.
Had he remained, K. said, he would have been expected to find a wife among the women assigned to his unit and to raise a family within the compound, which had schools, canteens and other facilities to keep employees relatively content for life. Most of the staff facilities were within the compound but above ground, and visible. K. said male recruits generally married after 10 years of military service, while women became eligible for marriage at age 24.
So complete was the secrecy that even employees inside had little idea of what was being produced.
"Some people said it was for chemical weapons. But everything was wrapped tightly with zinc so that we never really knew what was inside," K. said. "We weren't supposed to ask questions. We weren't supposed to wander around."
K.'s account is corroborated by the testimony of other defectors, who speak of secret military sites where the staff is virtually imprisoned.
"In these places, people have a lot of privileges. There is no problem with food and there are good schools, but they are like concentration camps too. You live in secrecy under constant suspicion," defector Lim said. "This is what happens if you volunteer for the chemical, nuclear or missile units."
The North Koreans' talent for concealment complicates efforts to forge a diplomatic solution to the current nuclear standoff. On Oct. 19, President Bush announced in Bangkok, Thailand, where he was attending a summit of Asian-Pacific leaders, that he would consider giving North Korea a written guarantee that the United States won't attack if Pyongyang verifiably and irreversibly dismantled its nuclear program. But how is complete verification possible when so much is hidden underground?
"Unless you are prepared to invade and occupy the whole country, you might never be able to find what you're looking for," said Pinkston of the Center for Nonproliferation Studies.
There is a precedent for inspecting North Korea's underground facilities that could be taken as a cautionary tale. In 1999, the United States demanded access to an excavation site called Kumchangri that was so large, it could accommodate an underground reactor and reprocessing plant. North Korea demanded $300 million in return -- which it received in the form of 600,000 tons of food aid from the United States and South Korea.
The team that conducted the inspection found the facility surrounded by thousands of North Korean soldiers, some doing kung fu exercises, others manning machine-gun nests at the entrance to the tunnel, according to Joel S. Wit, who headed the inspection team for the State Department.
"It was a little bit surreal," Wit said. "It looked like the set of a World War II movie."
What the inspectors did not find, however, was any evidence of nuclear weapons production despite a thorough scouring of the premises and sophisticated air and soil sampling. They left joking bitterly that the underground complex might be used instead as a mushroom farm or vast wine cellar for the private stock of North Korean leaders.
"We provided that food aid for nothing," said a South Korean security official, who asked not to be named. "We were all duped by the North Koreans."