Iran recently issued a press release boasting of its new underground missile storage sites. These “missile cities” are usually dug into mountains, sometimes by expanding existing natural cave systems. Iran revealed that in the last six years it had built three of these underground missile garages. Pictures have been released showing dozens of missiles stored in what appears to be underground settings. The recent construction program was made possible by the growing use of solid-fuel missiles, which do not require the time-consuming fueling process that older liquid-fuel rockets undergo before launch. For solid-fuel missiles, you need a truck capable of carrying, erecting the missile into an upright position and firing it. Each launch usually does some damage to the vehicle, which must undergo repairs or at least close inspection, before reuse. The Iranian missile bunkers are built mainly to prevent missiles from being destroyed by airstrikes. As has been demonstrated in Syria, Iranian missiles stored aboveground are extremely vulnerable.
The underground bunkers also have shortcomings. For example, the recent press release was mainly intended for foreign audiences. Reminding Iranians about the expense of building these underground facilities is not wise. Since 2017 Iranians have been increasingly outspoken about the amount of money spent on military projects, especially the $16 billion cost of the Syrian war since 2012,s very unpopular. More and more Iranians attribute their growing poverty and rising unemployment rates to all the money lavished on military efforts and these underground sites are very expensive to build and maintain.
Underground facilities also have drawbacks, aside from the high construction cost. The main problem is vulnerability to smart bombs and “penetrator bombs” designed to burrow deep into the ground, and through concrete, before detonating. Iran is aware of these and its latest press release also pointed out that the new missile bunkers were often built into mountains so that there are hundreds of meters of rock and earth between the missiles and the surface. Not mentioned was the vulnerability of entrances and air vents.
The main reason for all this use of underground facilities since the mid-20th century was the realization that air superiority enabled an enemy to freely bomb aboveground storage areas and military facilities in general. These underground bunkers proved useful against that type of attack using unguided bombs. This led to massive underground construction projects after World War II. The older unguided bombs were not accurate enough to do much damage to those underground facilities until the 1990s.
The development of smart bombs in the late 20th century provided other ways to deal with these fortifications. These bombs and missiles can be dropped in large quantities outside the range of air defenses and disable most of these underground facilities by destroying their entrances and air vents. The U.S. and South Korean air forces have invested in a lot of smart bombs for just that sort of attack. Many of these bombs are “penetrators” that burrow through many meters of earth and concrete before exploding. These are used against the largest and most important underground facilities. The Americans and South Koreans have trained to do this on a large scale in the event of a war and the North Koreans are faced with a countermeasure they never anticipated or prepared for.
Underground facilities have other vulnerabilities. Many of the underground factories near the Chinese border depend on hydroelectric dams and generators for power. Take these out and the facilities quickly become useless. Backup diesel generators don’t last long, especially if there are problems with the air supply. For underground storage facilities, the roads leading from the facilities road networks or nearby launch areas are vulnerable. Another factor is the late 20th century development of persistent surveillance using space satellites and high flying UAVs. This allows repair efforts to be spotted and quickly attacked. In this way, the underground storage areas become inaccessible and useless. At that point, the main function of these underground storage sites is to protect the missiles from destruction by airstrikes but not from being trapped and inaccessible underground.
Iran has long relied on underground facilities as protected spaces where they could build weapons and for ballistic missile and nuclear weapons development efforts. Iran and North Korea have cooperated on the design and construction of these facilities and the Iranians don’t have any solution for the smart bomb attacks either. Or, if they do, they are not publicizing it.
World War II (1939-45) saw extensive use of underground facilities because of the new threats from the air. Even before that, there was a threat from mass use of longer range and more accurate artillery systems developed early in the 20th century. During the 1930s the French built the Maginot Line along their German border. This was largely a series of tunnels and underground bases with aboveground cupolas for various weapons. The Germans built many underground structures during the war, including a large one for building their V-2 ballistic missiles. After the Korean War (1950-53) North Korea began building numerous underground facilities. What got this going was the extensive and effective use of American airpower against the North Koreans and their Chinese allies during the war.
Currently, there are believed to be about 10,000 underground facilities worldwide, including recently built ones for construction and/or storage of nuclear weapons. Most of these North Korea underground facilities are for storing weapons. These include tunnels dug under the DMZ (Demilitarized Zone) which forms its border with South Korea. Some of the tunnels that extended into South Korea were discovered and destroyed but South Korea believes there are still about twenty of them that extend into the five kilometer wide DMZ. These tunnels can accommodate up to 30,000 troops as well as vehicles and enable the troops to quickly exit near or in the DMZ if there were another war.
North Korea has built many smaller naval bases into mountains along the coast for small boats and mini-subs. Most of these tunnels are less than a kilometer long but in wartime would provide shelter for small subs and boats carrying commandos. These were based on the success of such seaside bunkers built and used successfully by the Germans during World War II to protect their submarine bases. These ship facilities can still be attacked and disabled, but damaging the protected facilities is much more difficult than simply bombing berths and dry docks built in the open. Even neutral Sweden built one of these facilities during World War II for small surface ships and recently put it back into use because of the growing Russian threat.
Most of the North Korean underground facilities (at least half) are for artillery and rocket launchers and are built close to the DMZ. In wartime, the artillery and rocket launchers emerge from tunnels, fire, and then withdraw back into the tunnels to avoid air attack or, for the rocket launchers, to reload. Many of these artillery tunnels are built on the reverse (facing north) slope of hills and mountains near the DMZ. Some of the heavier guns and rocket launchers are on rails and behind steel doors. The launchers or guns slide out on the rails, fire, then slide back in and the door is shut to avoid damage from air attacks. Many of these artillery tunnels are meant for bombarding South Korea’s largest city and capital, Seoul which is 50 kilometers south of the DMZ. Since the 1960s Seoul has expanded enormously and some of the suburbs are a lot closer to the DMZ.
There are also about 200 North Korean underground factories and weapons storage/repair sites. Most of these are near the Chinese border. The most recently built facilities, also near the Chinese border, are for the nuclear weapons program and assembling and launching larger ballistic missiles. There are also about ten underground living/working facilities around the North Korean capital Pyongyang. This includes at least 40 kilometers of underground roads and extremely well protected bunkers for the most senior leader.
During World War II the Japanese built more and more underground facilities on Pacific islands with the most extensive system built under the island of Iwo Jima. American marines suffered 26,000 casualties, including 6,800 dead, during five weeks of fighting to take Iwo Jima. Most of the 21,000 Japanese troops manning these fortifications fought to the death and only 216 were taken prisoner. The Japanese were observed building similar facilities on their home islands to oppose a planned 1946 invasion. It was estimated that the invading allied forces would suffer over half a million casualties dealing with these fortifications and fanatical resistance as demonstrated on the islands. The initial alternative was to completely blockade and bomb the home islands for another year, which would have left several million Japanese dead and many more starving. The successful alternative was the two atom bombs dropped in mid-1945, which compelled the Japanese to do the (to them) unthinkable and surrender unconditionally and immediately.
Use of smart bombs and penetrator weapons are not without their problems. Smart bombs dependent on GPS guidance can be forced, by jamming, to fall back on less accurate but unjammable INS systems. This means more smart bombs have to be used to destroy tunnel entrances and ventilation systems. A more insidious problem is penetrator bombs that work during tests but perform less well under combat conditions. A recent example of this was revealed in 2018 when U.S. Air Force ordered production of the BLU-137/B bomb as a replacement for the older BLU-109/B penetrator “Bunker Buster”. BLU-109 has been in use since 1985 and is usually mated with a Paveway laser-guidance kit like the GBU-27. Currently, the most frequently use penetrator bomb is this laser-guided 909 kg (2,000 pound) BLU-109/B. This “bunker buster” can penetrate five meters (16 feet) of concrete and even more of just dirt. This is accomplished using a 25mm thick steel cashing filled with 240 kg (530 pounds) of Tritonal. In the rear of the bomb is a time delay fuze.
Details of how the BLU-137/B is different were not revealed but it is known that the air force has been concerned for years with the number of BLU-109/Bs that failed to detonate after penetrating. This is not a new problem. Back in 2011, the U.S. Air Force spent $36 million to develop a new Hard Target Sensing Fuze for its large (BLU-109 and larger) penetrator bombs. The new fuze would also be reprogrammable by the pilot, while in the air. The pilot can specify how deep the bomb should go before detonating, as well as how many voids (levels of an underground bunker) to go through before detonating. A time delay can also be specified. The new fuze was able to survive a penetration force of 15,000 pounds per square inch (one ton per square cm). The new fuze is used with the BLU-109, 113 or 122 bombs. Improvements to the BLU-109 fuze and detonation system were not sufficient. As more BLU-109s were used against underground targets in Iraq and Syria (and Gaza, by Israeli aircraft) BDA (Bomb Damage Assessment) teams eventually got to target areas and confirmed what prisoner interrogation and other intel reports were describing as an “unacceptable” level of failed detonations. So the air force ordered a major redesign of its 2000 pound penetrator bomb and apparently tested it sufficiently to be satisfied the reliability problems were fixed and now ordered it into mass production. The BLU-137/B will be interchangeable with the BLU-109/B so that equipment on aircraft or guidance kits won’t have to be modified.
The failure rate of the BLU-109/B was not so high that could not be used. Instead, to ensure destruction of an underground target, multiple BLU-109/Bs would be used on certain targets. Many BLU-109/B targets were based on imperfect intel and were hit just to take the chance that the enemy bunker would be there and it would be destroyed. Many ISIL (Islamic State in Iraq and the Levant) members believed themselves protected by Allah when the penetrating bomb did indeed penetrate but did not detonate. Not exactly the sort of morale impact such weapons are supposed to have. If the BLU-137/B proves as reliable in combat as it did in testing Islamic terrorists will find their divine intervention has diminished when it comes to penetrating bombs. ISIL made extensive use of deep tunnels and bunkers. In Gaza, Hamas has been getting expert advice from Iranian tunnel builders on how to build more effective tunnels and bunkers. Israel had watched (from the air, and via spies on the ground) as Hezbollah used lots of its Iranian money to build underground bunkers in the areas of southern Lebanon that Israel withdrew from in 2000.
In 2012 Israel developed its own penetrator bomb design. This MPR-500.was for a 500 pound (227 kg) penetrator bomb. The MPR-500 can smash through more than a meter (39 inches) of concrete or four 200mm (8 inch) concrete barriers (floors or bunker walls) and then detonate. When the MPR-500 explodes it releases 26,000 fragments, which will wound or kill out to 100 meters. Satisfied with the effectiveness of the MPR-500 Israel went on to design 1,000 pound and 2,000 pound versions. It is unclear if the BLU-137/B design owes anything to the MPR line but Israel and the United States have freely exchanged design info and user experiences for decades and many American designs are heavily influenced and improved by Israeli technology.