Tunnel warfare
Particularly, the Chinese Communist forces or local peasant resistance used tunnel war tactics against the Japanese (and later the Kuomintang during Chinese Civil War). The tunnels were dug beneath the earth to cover the battlefield with numerous hidden gun holes to make a surprise attack. Entrances usually were hidden beneath a straw mat inside a house, or down a well. This allowed for flexible manoeuvers or exits.
The main disadvantage of tunnel war was that usually the Japanese could fill the holes or pour water in to suffocate the soldiers inside the tunnels. This proved to be a major problem but was later solved by installing filters that would consume the water and poisonous gases. It is said that there were even women and children who voluntarily fought in the tunnels.
The movie Tunnel War, which is based on the stories about fighting Japanese in tunnels, made tunnel warfare well known in China. More films were soon produced and adapted in the same setting.
After the war, the Ranzhuang tunnel site became a key heritage preservation unit promoting patriotism and national defense education. Being a famous war tourism site in China, it attracted tens of thousands of visitors each year. Most of the villagers were working in tourism service industry, an industry worth US$700,000 each year.

US–Japan War: The first to copy tunnel warfare were the Japanese themselves. In the battles of the Western Pacific, they would maximize their capabilities by establishing a strong point defense, using cave warfare. The first encounter of the US Marines with this new tactic was the island of Peleliu. The invading marines suffered twice as many casualties as on Tarawa, where the old Japanese tactic of defending the beach had been employed. The pinnacle of this form of defense, however, can be found on Iwo Jima, where the Japanese engineered the whole Mount Suribachi with many tunnels leading to defensive emplacements, or exits for quick counterattacks. Tunnel warfare by the Japanese forced the US Marines to adopt the "blowtorch and corkscrew" tactics to systematically flush out the Japanese defenders, one cave at a time.

Australia–Japan Conflict: In Australia, the demand for protection from air attack became more serious in the early 1940s when there was significant axis naval activity in Australian waters and when three Japanese midget submarines entered and attacked the Sydney Harbour in 1942. In Sydney in 1941, the Royal Australian Navy excavated a series of tunnels to shelter over 2,500 men working at the naval base from air raids, and as well as to transport guns and ammunition within the tunnels after the Australian government and people expected a Japanese invasion of Australia. There are other military fortifications in coastal Sydney that feature a tunnel warfare system, such as the Georges Head Battery (which was constructed in 1801 and was added to the New South Wales State Heritage Register in 1999), Lower Georges Heights Commanding Position (which was built in 1877 and became part of the Sydney Harbour defences, where the underground rooms and tunnels were used to store ammunition), Henry Head Battery (which was constructed in 1892 and was re-employed during World War II to defend the approaches to Botany Bay), the Middle Head Fortifications (a heritage-listed fort built in 1801), Malabar Battery (a coastal defense battery built in 1943) and the smaller Steel Point Battery.
Furthermore, in Wollongong, just south of Sydney, there exists the Illowra Battery and Drummond Battery. To the north of Sydney, in Newcastle, the Shepherds Hill military installations, a NSW state heritage-listed site, was built from 1890 to 1940 and consists of a former military gun battery emplacement, a 100 metres (330 ft) long tunnel and an observation post. As part of the strengthening of Newcastle's defense system, various new projects were undertaken at Shepherds Hill during WWII, such as accommodation for troops stationed. Fort Scratchley, which had close ties to Shepherds Hill, responded to an attack on Newcastle by a Japanese submarine in June 1942. This is the only place on the mainland of Australia known to have returned fire. The batteries at Shepherds Hill formed an integrated system with the batteries at Fort Scratchley, Fort Wallace at Stockton and at Tomaree on Port Stephens.

Japanese occupation of the Philippines: During the Japanese occupation of the Philippines, the Ilagan Japanese Tunnel was part of a military base built by the Japanese government as headquarters for its soldiers during World War II. 
In the Philippines campaign (1941–1942), Philippines President Manuel L. Quezon, General MacArthur, other high-ranking military officers and diplomats and families escaped the bombardment of Manila and were housed in Corregidor's Malinta Tunnel. Prior to their arrival, Malinta's laterals had served as high command headquarters, hospital and storage of food and arms. In March 1942, several U.S. Navy submarines arrived on the north side of Corregidor. The Navy brought in mail, orders, and weaponry. During the re-taking of the island by U.S. forces in 1945, Japanese soldiers who had been trapped in the tunnel after the entrance was blocked as a result of gunfire from USS Converse (DD-509) began committing suicide by detonating explosives within the tunnel complex the night of 23 February 1945. The collapsed laterals resulting from these explosions have never been excavated. 
During the Battle of Corregidor, the third lateral on the north side from the Malinta Tunnel's east entrance served as the headquarters of General Douglas MacArthur and the USAFFE. Malinta Tunnel also served as the seat of government of the Commonwealth of the Philippines. At the vicinity of the tunnel's west entrance in the afternoon of 30 December 1941, Manuel L. Quezon and Sergio Osmeña took their oaths of office as President and Vice-president of the Philippine Commonwealth in simple ceremonies attended by members of the garrison.

Korean War: On the Korean Peninsula, the underground war reached a massive scale. From experience in the Second World War, the US relied upon aviation. North Korean forces suffered heavy losses from air strikes which obliged them to construct underground shelters. Initially underground fortifications were built independently by individual units and their placement was chaotic. Subsequently, underground fortifications were united into a single large system. The length of the front was 250 kilometres (160 mi) while the length of tunnels was 500 kilometres (310 mi); for every kilometre of front, there were two kilometres of tunnels. A total of 2,000,000 cubic metres (71,000,000 cu ft) of rocks were extracted.
North Korea developed a theory of underground warfare. Manpower, warehouses and small calibre guns were completely housed underground making them less vulnerable to air strikes and artillery. On the surface, the many false targets (bunkers, trenches and decoy entrances to the tunnel system) made it difficult to detect true targets, forcing US forces to waste ammunition. Directly under the surface, spacious barracks were built, allowing whole units to be quickly brought to the surface for a short time and as quickly returned to shelter underground.
North Korea even created underground shelters for artillery. During bombing, artillery was rolled into bunkers located inside mountains. When a lull came, the guns were rolled back out onto a firing area, fired some shells, and rolled back into the bunker again. Unlike other examples of underground warfare, North Korean troops did not just remain in the tunnels. North Korean forces were sheltering in the tunnels from the bombing and shelling and awaiting US bayonet attacks. When US forces reached the ground in the area of the tunnels, chosen North Korean units would emerge to engage in hand-to-hand combat, taking advantage of their numerical superiority.
To this day, the North Korean strategy is to construct as many underground facilities as possible for military use in the event of a US attack. The depth of underground facilities reaches 80 to 100 m (260 to 330 ft), making them difficult to destroy even with the use of tactical nuclear weapons.
In the Korean War the tactic of tunnel warfare was employed by the Chinese forces themselves. "The Chinese resort to tunnel warfare, and the devastating losses to American soldiers, led to the sealing of tunnel entrances by United Nations Command. According to later prisoner of war interrogations, Chinese officers had killed a number of their own soldiers in the tunnels, because the latter had wished to dig their way out and surrender to the United Nations Command."
The Chinese People's Volunteer Army under General Qin Jiwei constructed an intricate series of defensive networks, which were composed of 9,000 meters (9,800 yd) of tunnels, 50,000 m (55,000 yd) of trenches and 5,000 m (5,500 yd) of obstacles and minefields.  This tunnel network proved its use in the Battle of Triangle Hill in October and November 1952, where, despite the United States Eight Army enjoying complete air and artillery superiority, the Chinese managed to keep the hill and inflict heavy casualties on the Americans.

Vietnam War: To maintain a full-scale guerrilla war in South Vietnam, camouflaged bases were used capable of supplying the guerillas for a long period of time. Throughout South Vietnam, there were secret underground bases that operated successfully. There are reports that every villager was obliged to dig 90 cm (35 in) of tunnel a day. The largest underground base was the tunnels of Cu Chi with an overall length of 320 km (200 mi). To combat the guerillas in the tunnels the US used soldiers dubbed tunnel rats.
Part of the Ho Chi Minh Trail was based in caves made of karst.