March 27, 2023: NATO nations have been studying operations in Ukraine to extract useful lessons for defending themselves in a future war. One important vulnerability was the ease with which airbases and the aircraft there can be destroyed by short-range ballistic missiles. Russia started the war with a large stockpile of these difficult-to-intercept missiles. Ukrainians quickly developed weapons and techniques for intercepting aircraft and cruise missiles, but Ballistic Missile Defense (BMD) had no ready solution. When the Russians used their ballistic missiles against Ukrainian targets, the target got hit. European NATO’s BMD plan was to have European air forces, reinforced by American warplanes and specialist support aircraft, quickly win air superiority against Russian airpower and air defenses and then smash Russian short-range ballistic missiles on the ground.

The Russian air force proved as inept as ever in Ukraine, but the Russians had their ballistic missiles. Ukraine had dispersed their aircraft to many temporary locations and this limited the damage done by the ballistic missiles. Ukrainian air defenses were able to make it difficult for Russian aircraft to operate and survive. European countries looked at their own air defenses and airbase vulnerability and realized they had problems. Since the 1990s European nations in general have spent a lot less on military capabilities. Currently, the Russian threat has been much diminished by the Ukrainian military and Western economic sanctions. The Russians make it clear that they will never give up on eventually absorbing neighbors who were once part of the Soviet Union. That list includes several post-1991 NATO members. That means the Russians will try again eventually. That gives European military planners and time to prepare for such an attack and reorganize to protect themselves from a surprise attack. It turned out that some non-NATO European nations had already taken precautions. These included air bases built into mountains and plenty of dispersed air fields. These were often little more than straight portions of highways where the air force had practiced shutting off ground traffic and converting that stretch of road into a temporary air strip.

European nations have to fix the many problems they created and tolerated after 1991. This included sloppy personnel standards and poor training and maintenance practices. A combat-ready military costs money and leaders who will make sure things get done. Ukraine was a wake-up call that may soon be forgotten. One way to avoid that is to undertake long term projects, like building the hardened bases using tunnels dug into mountains. There are plenty of examples currently and in the recent past showing how this is done.

For example, in 2019 Sweden reactivated its once secret Musko underground naval base. Built into the mountainous island of Musko, just south of the capital (Stockholm), the facility was never a secret to locals. Construction of the base began in 1950 and took 19 years to complete. Musko is connected to the mainland by a tunnel and a series of bridges between smaller islands. Parts of the base were usable in the 1960s before construction, which involved drilling, blasting and removing 1.5 million tons of rock, was completed. In addition to three docks for surface ships and submarines, there are also 20 kilometers of roads as well as numerous work, storage and living areas.

In 2004, after 35 years of use, ships were moved from Musko to other bases and the underground facility was largely closed. Some parts of Musko remained in use and details of the base were no longer top secret. But in 2019 the navy decided to revive the use of the entire base. While some ships are based there now, Musko mainly serves as the main headquarters for the Swedish Navy. There are several road exits from the complex and plenty of office space inside. All of this was maintained during the decade the navy halted its use as a major navy base.

Several other nations have, in the 20th century, built major underground military facilities. Switzerland, for example, has an airbase at Meiringen that is partially underground. While the main airstrip is outside the mountain, along a river, there are hangers and taxiways for aircraft built into the mountain. The Meiringen base began operations in 1941 and housed fighter aircraft vital for the defeat of any invading force. For a while, after World War II Meiringen was used mainly as an underground ammunition storage facility. In the 1960s more tunnels were dug for use as aircraft hangers and the base now serves F-18 and F-5 fighters. These aircraft can move directly into the main airstrip cavern wherein an emergency arrestor gear can be deployed to halt the aircraft if there is a problem during landing.

World War II saw extensive use of underground facilities. 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 manufacturing their V-2 ballistic missiles. After World War II and 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 over 8,000 underground facilities in North Korea, including recently built ones for work on and storage of nuclear weapons. North Korea has also dug tunnels 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. Most of the underground facilities (at least half) are for artillery and rocket launchers and are built close to the DMZ. In wartime, artillery and rocket launchers can 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 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 leaders.

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. The only 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 alternative was the two atomic bombs dropped in mid-1945, which compelled the Japanese to do the (to them) unthinkable and surrender.

The development of smart bombs in the late 20th century provided another way to deal with these fortifications. These bombs and missiles can be dropped in large quantities outside the range of air defenses and destroy or disable most of these facilities. 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. The 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.

Iran also relies a lot on underground facilities for building weapons and for their ballistic missile and nuclear weapons programs. 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.

Over a decade ago China built underground hangars for aircraft at an airport outside the city of Le Dong, on the southern island of Hainan (near Vietnam). There is also a submarine base that has underground docking facilities for nuclear and diesel-electric subs, created by tunneling into coastal hills. The underground facilities not only protect the boats from air or sea based attack, but enable maintenance and modifications to be done in secret. Same deal with the airport tunnels, which are also visible to anyone passing by on the ground. But there's nothing as compelling as pictures.