Since early 2015 reports have been coming out of North Korea about a new series of tunnels being constructed. Like everything else in North Korea tunnel construction is a state secret and it took a while for enough information to leak out (via business visitors hearing gossip or defectors with personal experience) so one could make some sense of this new mystery. It turns out the new tunnel system consists of escape tunnels for leader Kim Jong Un. These are being built at all the places where Kim spends a lot of time and for obvious reasons details are being kept secret. One tunnel, several kilometers long, leads to the Chinese border. Others lead to airfields or small ports. Apparently there are contingency plans for each tunnel. This would include what Kim and his entourage would do once reaching the exit as well as who would be in the entourage and who would be assigned to deal with all the details of getting Kim to safety after the “escape tunnel protocol” was implemented.
For decades North Korea has spent large sums on creating hundreds of kilometers of tunnels. Some of these went under the DMZ (the five kilometer wide demilitarized zone) that separates the two Koreas. But most of the tunnels were for protecting weapons and troops from air and artillery attack. Most of these were created by digging tunnels into the sides of mountains. Even when there are (like right now) power and fuel shortages work continues, slowly, mostly with manual labor. Another major use of the tunnels is to keep the air force operational despite the threat of American and South Korean attack using smart bombs. This led to expanding the network of underground parking and maintenance facilities for aircraft, as well runway extensions. These sheltered air bases begin underground, then exit the mountain and continue outside. Apparently the North Koreans have figured out that the Americans have now developed weapons that could quickly shut down these underground facilities, and keep them inoperable. That has led to further work to counter the new threat.
One of the key weapons for threatening North Korean tunnels is the U.S. Air Force 129 kg (285 pound) Small Diameter Bomb (SDB). The official story was that this GPS guided smart bomb was needed for urban warfare. The smaller blast (17kg/38 pounds of explosives, compared to 127 kg/280 pounds for the 500 pound bomb) from the SDB resulted in fewer civilian casualties. Friendly troops can be closer to the target when an SDB explodes. While the 227, 455, 911 kg (500, 1,000 and 2,000 pound) bombs have a spectacular effect when they go off, they are often overkill. The troops on the ground would rather have more, smaller, GPS bombs available. This caused the 227 kg (500 pound) JDAM to get developed quickly and put into service. But the smaller SDB was always a mystery, with many produced, but few actually used. Interestingly (especially to North Koreans) the SDB also has a hard steel, ground penetrating, front end that can punch through nearly two meters (six feet) of concrete. Not much use for that in urban warfare. But such a capability is very useful for taking out underground installations, particularly the entrances and air intakes. North Korea, for example, has twenty airfields with underground hangars for the aircraft. Usually tunneled into a nearby hill or mountain, the underground hangar allows fighters and bombers to quickly taxi out onto the runways and take off. Since North Korea doesn't have that many operational warplanes, it's believed that some of these "airfields" actually have long range rockets and ballistic missiles, mounted on trailers equipped to erect the missile into launch position and fire it off, in the underground hangars. The trailers are hauled out of the tunnels, onto the air field, the missile fired, and then the trailer is taken back inside to be reloaded. The North Koreans also have hundreds of other, smaller, underground facilities, close to the DMZ containing artillery and rocket launchers. These weapons are meant to be quickly hauled out and fired south.
Few details of Kim’s escape tunnels are made public like the exact location of the entrances and exits or the contingency plans. But with spy satellite much can be discovered. It is known that the workers who built the tunnels were taken in closed trucks and busses to and from the construction site. Only a few members of the Guard Command (also called the General Guard Bureau or Escort Bureau) know the details. The Guard Command is the elite security force for the Kim family and the most senior officials. Consisting of support (transport, supply, technical) and combat (bodyguards and several combat brigades) units, the force has over 50,000 personnel. Those selected to serve in the Guard Command are set for life, even after discharge. The Guard personnel are much better paid, dressed, fed and housed. Guard veterans have an edge in getting civilian jobs. This information about these tunnels is effectively kept secret by the Guard Command.