Am relatively sure Iran shall re-invent the bomb, just as the N Koreans managed. In Iran's case, Pakistan has been helping.

Believe Saudi Arabia shall be helped to re-invent the bomb as well.

I do not know what the Houthis and Persians are made of, but am aware that they have been at it for some time.

In moderation, up to a point, but beyond that point, might not matter.

But around the same time that these massive underground complexes were being built, tunnels also experienced a revival as a tool for insurgents. The pioneers in this revival of tunnel warfare were the Chinese during the Sino-Japanese War, especially during the fighting around the village of Ranzhuang in Hebei Province in 1937 and 1938. Chinese guerrillas dug nine miles of tunnels between houses in the village to foxholes on the battlefield, so that they could attack Japanese soldiers from the rear. The tunnel entrances and exits were usually located in a house or in a well, making it easier for guerrillas to enter and leave without being detected.

The Japanese soon caught on, however, and began filling the tunnels with water or even poison gas. The Chinese retaliated by installing filtering systems that drew off the water and the gas. This cat-and-mouse game -- which is typical of tunnel warfare -- continued until the Japanese finally withdrew. How important the tunnels of Ranzhuang were to the battle’s outcome is a matter of debate. To the Chinese, however, they are a monument to defiant resistance to the Japanese invader and, like the Maginot Line, are a major tourist attraction.

What the Japanese learned from the tunnel wars against the Chinese, however, would be invaluable in their fight against the U.S. Marines in World War II. They borrowed the techniques of hidden bunkers and emplacements connected by an elaborate network of tunnels, first on the island of Peleliu and then on Iwo Jima. There, they turned an entire mountain, Mount Suribachi, into a honeycomb of tunnels and bunkers lined with concrete, with multiple exits so that Marines clearing one end of the tunnels would find themselves suddenly under attack from the other end.

Clearing the Japanese tunnels was a grim business. Facing Japanese soldiers determined to fight to the death, U.S. Marines favored flamethrowers, explosive charges, and hand grenades (according to U.S. rules of engagement, poison gas was not an option). Marines on Peleliu suffered twice as many casualties as Marines fighting on Tarawa, largely because of the tunnels; the Marines on Iwo Jima were still clearing tunnels two months after the island had fallen.

There was method to the Japanese soldiers’ madness. They hoped that by inflicting as many U.S. casualties as possible -- and making the United States’ path to victory as slow, painful, and costly as possible -- they would deter Washington from attempting a similar full-scale invasion of Japan’s home islands. It worked, but not in the way the Japanese had hoped. In order to avoid an invasion, U.S. President Harry Truman chose to end the war by dropping atomic bombs on the Japanese cities of Hiroshima and Nagasaki.

But the most adept students of tunnel warfare during the Cold War were the Communist forces in the Korean and Vietnam conflicts. In Korea, underground warfare reached a new level of size and sophistication in the 1950s. To evade American air supremacy, North Korean and Chinese forces built underground fortifications so extensive that for every mile of military front on the surface, there were two miles of underground tunnels -- more than 300 miles in total. The tunnels were built largely by prisoners, who ripped out more than two million cubic meters of rock for structures that hid not only tens of thousands of soldiers and supplies, but entire artillery batteries that could be wheeled out of mountain caves to fire on South Korean or UN forces (and then drawn back in to dodge subsequent airstrikes).

The tunnels dug by Communist forces in South Vietnam were nowhere near as massive as the North Korean version, but they enabled the Vietcong to maintain a guerrilla war for years against a more numerous and better-armed foe. The biggest underground complex was the tunnels at Cu Chi close to Saigon, initiated during Vietnam’s Communist insurgency against the French colonial military in the 1950s. These tunnels extended some 200 miles toward the Cambodian border and came complete with ammunition storage, barracks, workshops, kitchens, hospitals, and even theaters for showing propaganda movies.

The U.S. military was so oblivious to the underground threat, at least at first, that in 1966 U.S. troops built a base camp -- a 1,500-acre compound housing 4,500 troops -- at Cu Chi, directly over the Vietcong tunnels. Black-clad guerrillas soon began organizing attacks on the base, popping out at night to blow up planes and steal weapons and equipment, including a tank, before disappearing into the darkness. The U.S. military responded by declaring the area around Cu Chi a “free fire” zone and pounded it with artillery, bombs, and even napalm in hopes of destroying the Vietcong. Yet the raids continued: from their tunnels, the Vietnamese guerrillas could wait out U.S. bombing raids and then prepare to strike again. The tunnels “were like a thorn stabbing the enemy in the eye,” a Vietcong officer later remembered, one that had become impossible for the U.S. military to remove. According to one historian, the tunnels had allowed the Vietcong to so deeply infiltrate the U.S. military installation that at one point, all 13 of the base’s barbers were members of the Vietcong.

When at last an Australian engineer revealed that the tunnels under the base were more extensive than anyone imagined, the U.S. Army realized what a hornets’ nest it was sitting on. The effort to clear the tunnels included teams of Australians, Americans, and New Zealanders dubbed “Tunnel Rats” who entered small surface access holes barely two feet wide, usually armed with nothing more than a flashlight, a few grenades, and a small pistol. What they found was a vast labyrinth of communication tunnels leading to caves and caverns built at four separate levels. With nerve and courage, the Tunnel Rats defied the claustrophobic and cramped conditions -- as well as booby traps, snakes, scorpions, hordes of bats, and angry Vietcong fighters -- to clear the Cu Chi complex from the inside. At the same time, B-52 airstrikes pounded the tunnels from above, causing many to collapse. Some 12,000 Vietcong fighters were killed in the Cu Chi operation, but the United States had barely started securing the tunnel complex when the country withdrew from the war. Today, even the Vietnamese honor the Tunnel Rats as the toughest, deadliest foe they ever faced. (The Israeli military has a similar unit, the Samoorim [“Weasels”], as part of the elite Yahalom combat engineers.) Although the Tunnel Rats could not save the U.S. mission in Vietnam, they did write one of the grittiest, if largely forgotten, chapters in the history of the U.S. Army.

In Vietnam, the tunnel digging stopped with the end of the war (although the Vietnamese revived their use during the Chinese invasion in 1978). Not so in North Korea. After the Korean War, Pyongyang’s appetite for tunnels increased. In preparation for a fresh invasion of South Korea, North Korea designed tunnel complexes across the demilitarized zone between the two countries. Between 1974 and 1990, South Korean authorities discovered four massive tunnels extending from North Korea under the border, each buried more than 100 meters under the surface and measuring two meters high and two meters wide -- wide enough for three North Korean soldiers to march through shoulder to shoulder (sufficient for a full division of North Korean troops, roughly 10,000 soldiers, to march through every hour). One of the tunnels emptied out just 30 miles from the South Korean capital of Seoul. South Korean authorities closed down the tunnels as they found them, but no one knows how many more may remain undiscovered.