Sweden has 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 building 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 about 8,000 underground facilities, 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, 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 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. 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 atom 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.