North Korea, world leader in digging military tunnels, will send thousands of engineering troops to Russian-occupied Ukraine this summer, reports South Korean TV channel Chosun. The destination would be Donetsk, a front line region with a 150-year history of coal mining.

“Three to four engineering brigades” would work in exchange for a Russian payment of $115 million, reports the conservative South Korean TV channel, citing a South Korean official. The tunnels-for-dollars deal comes after President Putin’s June 15 visit to Pyongyang,  his first since 2000.  

North Korean’s leader, Kim Jong-un, vowed “unconditional support” for Russia in the war with Ukraine as the two leaders signed a military mutual assistance pact. North Korea’s 1.2 million-man army is slightly larger than Russia’s 1.1 million-man army. Over the last year, North Korea has sent Russia dozens of ballistic missiles and as many as 5 million artillery shells, South Korea’s defense minister, Shin Wonsik, told Bloomberg last month. 

Military tunnels could change the dynamic of Ukraine’s front line, where drones now  scan battlefields day and night. In recent months, 80 percent of all damage was done by drones, the French army’s chief of staff, General Pierre Schill, told reporters last month at a Paris arms show.  

Since October, Russian troops have won two battles by reopening and expanding abandoned tunnels in Donetsk. On the Korean peninsula, North Korea has built tunnels large enough to pump 30,000 troops an hour into South Korea.

“There is information that four engineering brigades may be coming here from North Korea. How dangerous are they? During the Korean War, tunnel warfare was used very actively,” Major Yehor Checherynda of the Ukrainian Army warns, speaking Monday on Ukraine’s Espreso TV. “Ukrainian Defense Forces must take this into account to prevent such operations on Ukrainian territory.”

North Korea increasingly exports its tunneling expertise. In the late 1960s, it built in the mountains north of Hanoi a secret underground headquarters complex for North Vietnam’s government. Between 2003 and 2006, it built tunnels under the new capital, Naypyidaw, for Myanmar’s military dictatorship.

More recently, North Korean military engineers, with funding from the Iranian Revolutionary Guards Corps, helped to design and build “Hezbollah Tunnel Land,”  hundreds of miles of tunnels that honeycomb southern Lebanon. Israeli observers say wheelbarrows and dirt piles are sometimes positioned at the mouths of decoy tunnels, according to Israel’s Alma Research and Education Center 

Korea Mining Development Trading Corporation, a major North Korean arms exporter, oversaw much of the tunnel work, according to a lengthy study posted last month on Israel Hayom news site, “Hezbollah’s vast tunnel network in Lebanon is in a completely different league to Hamas.” Below Gaza, North Korean techniques and possibly advisors helped Hamas build the “Gaza Metro,” a maze  of tunnels believed to total 300 miles below the 25-mile long Strip.

During the Korean War, North Korea’s military went underground in response to the massive American bombing campaign. By the time of the 1953 Armistice, American bombing had destroyed 85 percent of all structures in North Korea, according to research by Washington Post reporter Blaine Harden. The State Department official for East Asia, Dean Rusk, said America bombed “everything that moved in North Korea, every brick standing on top of another.”

To protect military bases and factories from American B-29s, North Korea started to dig. Scattered across the country, military engineers have built as many as 8,000 underground facilities to shelter the regime’s leadership, according to South Korea’s military journal “Joint Chiefs of Staff.”

Near the DMZ, North Korea has dug 800 bunkers capable of concealing up to 2,000 soldiers each. Tunneling into mountains facing Seoul, 30 miles to the south, North Korea has dug hundreds of caves designed to allow artillery cannons to fire on South Korea’s capital — and then pull back for reloading.

“The North Korean People’s Liberation Army Air Force is believed to have three different underground air bases at Wonsan, Jangjin and Onchun,” a Northeast Asia defense analyst, Kyle Mizokami, writes for the National Interest that North Korea’s “underground base at Wonsan reportedly includes a runway 5,900 feet long and ninety feet wide that passes through a mountain.”

In South Korea, the most visible manifestations of North Korean tunneling expertise are three deactivated tunnels that are open to tourists. Wide enough to allow jeeps, the tunnels are lined with concrete, have powerful air ventilation systems and sometimes go as deep as 500 feet.

The first tunnel was discovered in 1974 when a South Korean patrol noticed steam rising from the ground. In a subsequent exploration of the tunnel an American Navy commander and a South Korean Marine Corps major were killed by a booby trap explosive.

Over the next 15 years, the South Koreans discovered three more major tunnels under the DMZ. The most effective detection methods were information from defectors and well drilling to detect voids. In Donetsk, the North Koreans will find a strong mining culture. Until Russia first occupied the area in 2014, mining accounted for one quarter of all jobs.

In 1913, the Donbass region produced 87 percent of Tsarist Russia’s coal. In an area riddled with mines, the Soviet Union actually used one for a nuclear bomb test. On September 16, 1979, a small bomb was detonated 3,000 feet below the surface of the Yunkom, or Young Communard, mine.

More recently, Donetsk mining expertise was on display in two battles, at Avdiivka last fall, then at Pivnichne where Russian soldiers tunneled below Ukrainian positions. Popping out undetected, they carried the day. Reflecting the value Moscow puts on tunnel operations, the Russia officer who led the Avdiivka operation, Anton “Zima” Morozov, 28, was recommended for the Kremlin’s highest honor: Hero of Russia. 

On Sunday, Russia’s defense ministry described its soldiers’ recent use of a tunnel to overcome Ukrainian defenders at Pivnichne, a village, near the disputed city of Toretsk.

“The detachment’s servicemen secretly cleared and used a tunnel more than 3 kilometers long along the Seversky Donets Canal, and entered the rear of a well-fortified stronghold with long-term firing points and underground shelters,” the defense ministry said on its Telegram channel. 

“Through the tunnel,” the ministry added, “the servicemen established a supply of ammunition, weapons and food for the assault troops. Using the element of surprise, the unit’s servicemen developed their success and completely captured the stronghold, forcing the enemy to surrender or abandon their positions and retreat.”

As the largely frozen frontlines in southeast Ukraine increasingly look like World War I standoffs, the Russians “have started using the tactic of digging tunnels,” Ukrainian military spokesman Anton Kotsukon told national television. “They’re digging them close to our positions,” he said. “First, for concealment. Second, they can then unexpectedly emerge close to our positions.”

After Russia and North Korea signed their mutual defense pact, South Korea’s national security advisor, Chang Ho-jin, told reporters that South Korea was considering sending lethal military aid to Ukraine. With half a century of experience in detecting North Korean tunnels, South Korea could aid Ukraine by sharing the techniques of its highly successful Tunnel Neutralization Team.