Recent analysis of satellite photos of the west coast North Korean long-range missile base at Sohae revealed that the above ground missiles assembly and fueling facilities were only part of the complex. Construction at Sohae began in the 1990s and became more obvious (from space) after 2000. The base finally became operational in 2012. Recently the satellite photos revealed considerable previously undetected construction underground. There are apparently facilities down there, out of sight, for assembly and fueling of large (ICBMs or satellite launcher) rockets. The latest satellite photos showed construction of rail lines in the base that are extensive enough to handle moving an assembled rockets from underground assembly areas and into position for firing. What this all means is that North Korea can carry out launches with little advance warning. When the rocket assembly and fueling took place above ground satellite photos gave weeks of warning. Now a rocket can be brought out and placed in launch position within a few days.
There were signs that something was happening with North Koreas large force of tunnel builders. Since about 2000 there was a pronounced slowdown in North Korean work on underground air bases. Part of this was thought to be the result of growing energy shortages up north, and the frequent blackouts. It's not just electricity that's been in short in North Korea over the last decade, it's everything. That includes construction equipment, especially the specialized stuff needed for digging tunnels into the sides of mountains. Visible work on some of the usual underground facilities continued, slowly, mostly with manual labor. This expanded 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. It was thought that this was the North Korean response to a new American penetrating smart bomb that could quickly shut down these underground facilities, and keep them inoperable. That includes the Sohae facility.
One of the key weapons for doing this 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.
But the SDB also has a hard steel, ground penetrating, front end that can penetrate 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 South Korea border, containing artillery and rocket launchers. These weapons are meant to be quickly hauled out and fired south.
That's where the SDB comes in, but the U.S. Air Force isn't saying much about it. The SDB would be the ideal weapon for launching a surprise attack on North Korean underground facilities, both the airfield hangers and the artillery bunkers. American B-2 and F-22 aircraft can dodge North Korea radar and drop a lot of SDBs all at once. A B-2 can carry over 200 SDBs. An F-22 can carry eight, and still protect the B-2s against any North Korean fighters that might have been in the air at the time of the attack. A half dozen B-2s carry over 1,200 SDBs, which is sufficient to cripple North Korean air defenses and twenty underground air bases. A few dozen F-22s carry another 300 SDBs to hit smaller, spread out targets. The SDBs not only shut down the entrances to the hangars, but also blow deep holes in the airfields. While North Korea has thousands of troops trained and equipped to quickly come in and clear the hangar entrances and repair the airfields, they are not quick enough to do so before unstealthy B-1s and B-52s come in with more smart bombs. These include cluster bombs, carrying thousands of small booby traps that explode when stepped on or rolled over by vehicles or engineering equipment.
The Sohae underground complex is also vulnerable to the SDB and larger penetrating bombs but the problem with Sohae is that it could be used to launch a nuclear armed long range ballistic missile that could hit the West Coast of the United States.