During the Korean War, Kim Il Sung and his army witnessed the terrible and far-reaching ability of the Allied air forces to knock out military and economic targets from the sky. The northern part of the Korean Peninsula had been turned into Korea's industrial and most developed region by Japan during their 35 year occupation, but by the end of the three year Korean War, upwards of 85% of all buildings in North Korea had been completely or partially destroyed. The Korean War saw more tonnage of bombs dropped than in the whole of the Pacific Theater of WWII.
Kim Il Sung's expectation that Korean War II was imminent, and based on the lessons he learned during the first war, led to him commanding that the whole country be fortified, saying in 1963, "we must dig ourselves into the ground to protect ourselves".
At this point, I want to sound a note of caution to those who like to let stories of North Korean mysteries run away with them. There has been plenty of speculation about the true extent of North Korea's underground infrastructure, with some going so far as to claim that most of their military bases are actually buried beneath the ground. The truth is, while both small and vast underground sites exist across the country, commercial satellite images do not support the idea that there's basically a parallel country underground.
There are also scores of stories about secret underground escape routes that connect Pyongyang and major palaces with underground rail lines that would allow the Kim family and top officials to be spirited away to the far north of the country, and even to China.
While the exact purpose of each and every one of the sites is impossible to ascertain by simply looking at them from the air, most tend to fall into a handful of categories. There are a few major underground facilities, such as the Punggye-ri Nuclear Site, the Panghyon underground aircraft factory, and Kim's hardened helicopter base (which is part of a much larger underground command and control base). Most of the others can be classified into these: the underground factory or laboratory, the unspecified underground facility (many with protective berms at tunnel entrances), underground facilities at Navy and Air Force bases (excluding HARTS and storage bunkers), and basic tunnels (either individual tunnels or usually in groups of three).
The following set of images shows examples of various types of underground facilities. The satellite images used cover a large range of dates, but they were chosen because they best showed the sites in question.
The Hagap Facility in Chagang Province (40.081644° 126.189346°) is an example of a major underground site. Its exact purpose is unknown, but the two main theories are that it's either part of North Korea's nuclear program or a secured storage site for important government archives. Construction of the site has been ongoing for decades, punctuated by periods of inactivity, but since 2016, work has been steady.
Ryoho-ri Underground Naval Base (39.876051° 127.785328°)
Tonghungsan Machine Plant (39.953611° 127.546918°)
While North Korea's engineers are capable of constructing large underground factories, they're still limited by the technology, education, and other factors that they have access to. North Korea is well known for their "speed campaigns" and for constructing large projects rapidly. Lack of enough materials due to sanctions and additional factors means that some of these projects end up shoddily built, with problems arising often and even occasional building collapses. These failures can also extend to military sites.
The following site is a coastal artillery position in North Hamgyong Province (41.896007° 129.950076°) that suffered a collapse, largely destroying the site.
Area before the collapse.
Area after the collapse.
North Korea built one of the deepest subway systems in the world, and they have a history of building underground facilities not only within their own borders but also around the globe.
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--Jacob Bogle, 2/17/2021