North Korea has been spotted (from space) building missile silos next to a dormant volcano (Mount Paektu) on the Chinese border. In fact, half the volcano is in China, where it is a popular tourist destination for South Koreans. That’s because Koreans and Manchus (as in Manchuria, the native people of northeast China) both consider Mt. Paektu as a sacred place where their tribes originated thousands of years ago. North Korea has put the silos for their long range (2,000-3,000 kilometers) ballistic missiles up there because that part of North Korea is a triangle, surrounded on two sides by China. This makes it difficult for the Americans to launch air attacks without entering Chinese territory and makes it easier for North Korean anti-aircraft forces to defend against cruise missiles. On the down side, Paektu is a dormant volcano that is active (lava flows and the like) about once a century. The last time it erupted was in 1703 and an eruption in the late 10th century blew the top off the mountain and created the current 4.5 kilometers wide crater lake. Volcanologists consider Paektu capable of another major eruption but North Korea considers that less likely than an American air attack. So the silos stay, despite the risk of destruction by an eruption or lava flows.
Before all these silos were built North Korea planned to keep its long range ballistic missiles mobile and launch them from any number of launch sites (a flat field where the missile could be fueled and the guidance system programmed before launch). Bad weather could complicate the use of mobile launchers (washing out bridges or blocking roads with snow). The quality of North Korean roads has also declined sharply (from lack of maintenance) in the last decade. Then there is the increased American surveillance (from satellites, U-2s, and high-altitude UAVs) that makes mobile missiles more vulnerable to air or missile attack. Silos can also be attacked from the air, but in a war the more numerous and shorter range ballistic missiles to the south would also be subject to air attack, as these missiles would be aimed at the South Korean capital.
North Korea apparently believes that silos protected by a sacred volcano are a worthwhile investment to ensure that some of the long-range missiles will get launched during a crises.