Hopefully, the infiltration tunnels will remain merely another footnote in the conflict between the two Koreas that will never be used in an actual conflict. Nonetheless, they remain a testament to the great lengths North Korea has undertaken to undermine the defenses of its southern neighbor, and the sense of paranoia and uncertainty that can inspire in civilian and military leaders alike.

On November 15, 1974 a squad of South Korean soldiers stationed near Korangpo-ri, on the Korean demilitarized zone, noticed steam rising from the earth’s surface near to where they were camped out.

(This first appeared several years ago.)

Lt. Col. Michael Wikan, who served as a G-3 operations officer in Korea, recounted what happened next in the book Espionage and the United States During the 20th Century, by Thomas Murray:

one of the ROK soldiers with sharp eyes noticed the heat waves rising from the ground and went to the location to investigate. When he heard voices up from the small hole, he fastened his bayonet to his rifle and probed—and more earth crumbled. When he fired his rifle into the hole, a volley of North Korean bullets flew back at him from the tunnel—and then silence. The South Korean squad reported the incident and dug a hole to open the tunnel, but no one entered.

South Korean troops across the 160-mile-long DMZ had overheard explosions and subterranean activity for over a year, and noticed heavy digging equipment moving around on the North Korean side of the border. This, however, was the first tunnel to be located—but nobody was in a rush to find out what was inside.

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Five days later, the United Nations Command dispatched U.S. Navy Cdr. Robert M. Ballinger and Marine Maj. Anthony Nastri to inspect the tunnel. Because protocol required that they remain unarmed, they were escorted by troops led by Korean Marine Maj. Kim Hah-chul. All three officers were veterans of the Vietnam War. Wikan continues:

Bob lowered himself into the hole first, followed by Tony. Less than a minute later [at 1:20 PM], a huge explosion went off that killed Bob instantly. The South Korean soldiers quickly pulled Tony out of the hole.

We could never determine the exact type of explosive device that was involved, whether a booby trap, mere blasting materials, or a command-detonated mine. I have always believed they dug some blasting explosives into a sidewall and electrically detonated it from a distance.

Major Kim was also killed in the blast, and five U.S. troops and one Korean were wounded.

Later inspections revealed the so-called First Tunnel of Aggression had been lavished with concrete-slab walls, electrical lighting, weapon-storage areas and sleeping accommodations. There was even a railway with carts installed. The tunnel was over two miles long, a third of which was on the South Korean side of the border, and had space enough for two thousand soldiers to traverse it per hour.

According to a South Korean defense white paper, Kim Il-sung had ordered the tunnel building campaign in a meeting on September 25, 1971, proclaiming somewhat optimistically, “One tunnel can be more powerful and effective than ten atomic bombs put together and the tunnels are the most ideal means of penetrating the South’s fortified front line.” The tunnel-building campaign was supposed to be completed by 1975.

The underground passageways could be used in peace to funnel agents into South Korea or in war to enable commando and light infantry units to bypass the heavy fortifications of the DMZ, cutting South Korean logistical lines and creating a “second front line” that reinforcements would have to fight through.

Four months after the first tunnel was discovered, a much larger tunnel was discovered thirteen miles north of Cheorwon, at the center of the demilitarized zone. Subterranean explosions had been heard there for over three years. Finally, South Korean troops combing the area dug an intercept tunnel in March 1975. This second tunnel penetrated one mile over the DMZ and measured nearly two by two meters in size, with a vaulted ceiling—ostensibly large enough to accommodate small vehicles and artillery, or to disgorge thirty thousand troops per hour under the border into South Korea. A large troop-assembly area was carved out, and there were three separate exits to facilitate a rapid flow of troops.

The Third Tunnel of Aggression, the most famous, was discovered in 1978, just twenty-seven miles north of Seoul, within a couples miles of the “truce village” of Panmunjom and the American Camp Kitty Hawk. South Korean troops had been searching the areas for several years based on the account of a defector named Kim Pu-song. The tunnel was finally located on June 10, when North Korean tunneling activity caused a jet of water to burst open the covering of an older South Korean bore hole. This Third Tunnel, which was 240 feet deep and penetrated four hundred meters past the DMZ, was of similar design to the second tunnel, and was well positioned for launching a surprise attack on the South Korean capital.

North Korea initially denied any responsibility for the tunnel, which violates the armistice it signed in 1953, then claimed it was dug for coal mining, a fact belied by the granite foundations. Today the Third Tunnel has become a popular tourist attraction.

The fourth and (so far) final tunnel was located nearly five hundred feet underground, near Haean on the eastern edge of the DMZ—the site of the infamous Punchbowl, a huge bowl-shaped valley that saw numerous battles during the Korean War. An intercept tunnel exposed the tunnel in March 1990, with media in attendance to record the event. You can see the locations of all four tunnels in this map.

A Korean defense white paper from 1990 claims, “It is almost impossible to detect a two-meter-wide tunnel some hundreds of meters under the ground.” It is commonly estimated that that there are between sixteen and twenty more infiltration tunnels that have escaped detection, based on photographic intelligence, defectors’ accounts and overheard demolition activities. For example, the defector Kim Pu-song claimed he had seen no fewer than nine tunnels while working as a surveyor. However, no additional infiltration tunnels have been located since 1990.

Nonetheless, the specter of subterranean infiltrators has inspired some South Korean civilians to privately fund their own tunnel searches, to the annoyance of the military. For example, a BBC article from 2012 tells the story of a pastor who lost much of his congregation due to his obsession with tunnel hunting, while a 2014 CNN story relates how retired South Korean general Hahn Sung-chu grew convinced there was tunneling activity under an apartment building in Seoul.

South Korean government officials insist long-distance tunnels are unlikely, due to extensive groundwater they would have to traverse. The CNN article quotes a defector claiming tunnel building continued well into the 1980s and would have sought to form links with Seoul’s sewer system, for potential use by disguised operators.

The U.S. military is also preoccupied by tunnel-fighting scenarios on the Korean Peninsula—which, in the event of a war, could also involve attacks on extensive tunnel networks in North Korea, including underground nuclear and chemical-weapons facilities and Hardened Artillery Sites (HARTS). Reports early in 2018 revealed that the U.S. military has expanded its tunnel-warfare training program, which is currently focused on the Eighty-Second Airborne and 101st Air Mobile divisions, having recently completed courses for troops in the First Cavalry.

The extent of the remaining infiltration tunnels and the role they would actually play in North Korean military strategy remains a mystery. South Korea currently possesses far superior conventional-warfare capabilities, so North Korean strategy has shifted away from invasion to a deterrence strategy using ballistic missiles, heavy artillery and chemical and nuclear weapons to threaten terrible damage to civilian targets.

Still, North Korea has also long emphasized infiltration behind the DMZ, and maintains around two hundred thousand special-operations troops, the largest special-operations force in the world. In the event of a war, these elite soldiers could be inserted by low-flying transport planes, stealthy submarines and submersible speed boats, American-built MD500 helicopters and, of course, the infiltration tunnels. Such missions might be effectively suicidal, but commando teams could spread considerable chaos and impose major delays before they were eliminated.

Hopefully, the infiltration tunnels will remain merely another footnote in the conflict between the two Koreas that will never be used in an actual conflict. Nonetheless, they remain a testament to the great lengths North Korea has undertaken to undermine the defenses of its southern neighbor, and the sense of paranoia and uncertainty that can inspire in civilian and military leaders alike.

Sébastien Roblin holds a master’s degree in conflict resolution from Georgetown University and served as a university instructor for the Peace Corps in China. He has also worked in education, editing and refugee resettlement in France and the United States. He currently writes on security and military history for War Is Boring.