Yes, this is true.
Once located, there are three ways of dealing with the sites. The first and safest way to deal with them is to bomb them from above. This presents the least risk to allied forces, but it will also prove difficult to determine whether air or artillery strikes have had good effect. The use of bombs or artillery shells may cause cave-ins that prevent allied forces from entering an underground complex and exploiting any intelligence found inside.
North Korea, one of the most secretive countries in the world, is no stranger to building underground military facilities. Whether a tunnel dug under the demilitarized zone designed to pass thousands of troops an hour, or bunkers to accommodate the regime’s leadership, North Korea has built extensive underground facilities designed to give it an edge in wartime.
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One of the earliest examples of North Korean underground engineering was the discovery of several tunnels leading from North Korea under the demilitarized zone to South Korea. The first tunnel was located in 1974, extending one kilometer south of the DMZ. The tunnel was large enough to move up to two thousand troops per hour under the DMZ. A U.S. Navy officer and South Korean Marine corporal were killed by a booby trap while investigating the tunnel. Thanks to a tip from a North Korean defector, an even larger tunnel was discovered in 1978, a mile long and nearly seven feet wide.
Since then at least four tunnels have been discovered, with reinforced concrete slabs, electricity for lighting and fresh air generation, and narrow railway gauges to shuttle dirt and rock back to the tunnel entrance. Collectively, the four tunnels would have likely been able to move a brigade’s worth of troops an hour under South Korea’s defenses.