The North Koreans are tunneling again.

Call them the diggers of Asia - the guys who build huge complexes and send aggressive shafts of cleared-out dirt under the demilitarized zone with South Korea.Once, the best in the area were the North Vietnamese, whose prowess impressed the United States during the Vietnam War - and cost many American lives.

The North Koreans were also impressed and increased the clandestine digging activity they'd begun during the Korean War. The proliferation of U.S. spy satellites caused them to redouble their digging in the 1960s.

So it's not surprising that intelligence reports focusing on a huge nuclear site about 25 miles northeast of Yongbyon find that most of the activity is underground - and that more than 15,000 North Koreans have been deployed to keep carving out the ground so prying, spying silicon eyes can't see it from above.

"You can't miss the employment aspect of all this," confided a North Korean defector to our associate Dale Van Atta. "Kim Jong Il knows it's a good way to keep his military busy - construction. His father once told him that the real reason for the pyramids in Egypt was so the pharaohs had something for idle, restless slave hands to do, other than revolt."

The late "Dear Leader," father of the current tyrant, Kim Il-Sung, became a convert to underground facilities and tunnels in 1970, when the military stepped up what was previously a moderately defensive program. Since that time, at least a dozen airfields and other military bases have significant underground facilities supporting them.

"Extensive construction of huge underground aircraft support facilities as well as hardened hangarettes and revetments has taken place," says one Defense Intelligence Agency estimate, classified "Secret." It adds: "Virtually all major air bases have hardened or underground facilities, or both."

The North Korean navy is equally well protected. At most of its "principal naval installations," according to another classified DIA report, the North Koreans have "constructed underground naval facilities," including "underground (or sheltered) berthing" for the country's fleet.

To support its army, the DIA continues, "since at least 1970, North Korea has been engaged in a program to expand its materiel storage capacity. (This includes) expanding its underground facilities, including sites for ammunition, POL (petroleum-oil-lubricants), and food storage."

The DIA has estimated that there are "several hundred storage facilities located within 75 kilometers of the DMZ (Demilitarized Zone)." They also reckon that the North Koreans have stored, often in underground facilities, more than "4.5 million barrels of POL and well over 1 million metric tons of other supplies, including ammunition" - which is enough "to wage intensive combat for at least 30 days, perhaps several months."

At least a half dozen of these huge tunnels, some big enough to drive tanks through, have been discovered by the U.N. Command. The DIA has warned that "until all are located and neutralized, North Korean-built tunnels represent a potentially significant military threat to the South."

Besides the tunnels, the Kim family has also invested heavily in spying on its South Korean neighbors. One DIA report noted that "construction and use of obstacles and mockups of ROK (South Korea) DMZ-type defensive barriers throughout most of the North Korean corps areas illustrates the offensive nature of some ground force training."

After noting the military training on mockups, the DIA report elliptically notes that "mockups of ROK urban areas, including Seoul, reportedly are located in North Korea."

Most U.S. intelligence analysts familiar with the North Korean military consider it highly probable that Kim has safeguarded his nuclear bomb-making with underground complexes as yet undiscovered by the West.

The North Korean penchant for excavation - and lying - should give the Clinton administration serious pause in accepting any assurances that they are complying with nuclear-related accords.