Introduction & Details
Tunnel warfare, an otherwise niche aspect of battlefield tactics, has grabbed the world’s attention since the Hamas attack of 7 October 2023. While this report does not focus on this conflict specifically, tunnels and their usage in fighting have a long history, and their complex nature requires closer examination.
Tunnels range from sophisticated structures with ventilation and shoring to crude hand-dug passageways. They are often multipurpose and can be used for smuggling weapons and people, storing ammunition and supplies, or providing cover and concealment during an offensive. Other fully underground structures such as civilian infrastructure (sewers, subway tunnels, etc.) can also be used by insurgent groups. This report outlines the significance of tunnels to insurgent groups for purposes such as cross-border infiltration, attacks, and more broadly, reducing the asymmetric military advantage of a state security force.
Past Uses & Renowned Cases
Some of the most extensive and best-known uses of tunnels for insurgent warfare stem from the Vietnam War. Spanning around 250 kilometers, the North Vietnamese and Viet Cong troops hand-dug a vast network of tunnels and underground facilities for housing, medical centers, storage, and other purposes. Digging occurred during the monsoon season when the soil was soft, and the iron and clay in the earth caused the walls to harden into a concrete-like substance upon drying. These tunnels provided the Vietnamese with the ability to stealthily move through contested areas and launch surprise attacks. Detecting and destroying such structures proved difficult for the U.S. and allied forces who deployed a variety of methods ranging from infantry armed with flashlights and handguns (so-called tunnel rats) to saturation bombing via B-52 bombers (1).
In Afghanistan, the use of subterranean spaces to combat invading forces has a history dating back to Alexander the Great. This is due to the natural cave formations in the Hindu Kush mountains. The naturally occurring structures were, in part, further developed by Afghan Mujahideen forces during the Soviet occupation of the country. Unlike in Vietnam, these structures are far more difficult to construct due to the limestone and crystalline rock that is found in the Afghan mountains (2). However, Afghanistan does have substantial networks of qanat or karez tunnels. These describe a still-used ancient system of transporting water through underground aqueducts. Being underground they have been used to shelter and move fighters or to store weapons caches throughout history but most recently during the U.S.-led invasion of Afghanistan (3).
The tunnels dug by Hamas in Gaza became front-page news in the wake of the October 7th attacks. However, these tunnels are not new and were previously used to abduct an Israeli service member in 2006. In 2014, Operation Protective Edge initially aimed to prevent rocket fire from Gaza and quickly shifted its primary objective to detecting and destroying newly discovered tunnels. The operation resulted in the destruction of 32 cross-border tunnels (4). Similar tunnels dug by Hezbollah with assistance from North Korea have crossed Israel’s northern border with Lebanon. The 2019 Operation Northern Shield aimed at finding and eliminating these (5). Since 7 October 2023, the tunnels have been alleged to house hundreds of hostages, while Israeli airstrikes have claimed to be targeting the tunnel infrastructure in Gaza. Footage from both sides has also been released showing the use of tunnels during the on-the-ground fighting (6).
Purpose of Use & Details
Insurgent groups are often pushed underground by the technological overmatch of state militaries (7). The subterranean space is especially useful to counter state ISR (intelligence, surveillance, and reconnaissance) and aerial strike capabilities. Insurgent groups may use tunnels for movement, storage, and communication with significantly less risk of detection. Additionally, a nonstate group may move their command-and-control centers underground during periods of intense fighting to maintain their combat ability. This was the case with Hamas, who housed senior operational leadership underground during Operation Protective Edge. Doing so increases a group’s defensive resilience when facing enemy attacks (8).
In addition to the evasive and defensive nature of tunnels, they also offer significant offensive capabilities. So-called “attack tunnels” allow for clandestine staging prior to an attack, coordinated strikes on multiple targets, and flanking of the enemy from the rear (9). In Vietnam, tunnel systems were extensively used to carry out ambushes before quickly withdrawing or infiltrating enemy bases (10). The psychological impact of potentially facing an attack at any moment without knowing where it could come from cannot be underestimated.
Finally, tunnels and other underground facilities require significant resources to detect and destroy. Air and artillery strikes may only obscure tunnel entrances without destroying the underground structures or eliminating combatants in them (11). Detecting and destroying tunnels is also costly in terms of the manpower, equipment, and explosives it requires (see 'countermeasures' section). Israeli manoeuver units tasked with finding tunnel entrances in Gaza, for example, faced stiff opposition above ground while engineers were often only able to destroy the first or last 200 meters of a tunnel which can quickly be re-excavated (12).
Technical Analysis
In terms to the environment favored for the use of tunnels in insurgent warfare, we must consider the various categories of underground structures. According to a U.S. Department of the Army Techniques Publication, there are three such categories. As seen in Table 1-1, Category 1 includes Tunnels, Caves, and Natural Cavities, Category 2 is Urban Subsurface Systems, and Category 3 Underground Facilities (13).
For insurgent groups, the first two categories are the most relevant, as the facilities described in Category 3 are unlikely to be under insurgent control. While the use of Category 2 structures has seen less use in recent times, structures such as sewers saw use in partisan fighting during World War II (14). Category 1 structures can be implemented in urban areas, however, given the risk of detection during construction, they likely require the insurgent group to exercise some control at the surface level. In rural areas, their use is heavily dependent on soil composition as seen in the systems found in Vietnam or Afghanistan.
While tunnels have many benefits for clandestine groups, fighting in tunnels, despite its many advantages for the defenders, is nightmarish for either side. Subterranean structures increase the tactical hazards of combat by compounding the enemy threat due to limited visibility, movement, tracking, and communications (15). Additionally, the walls and ceilings provide more options for placing traps or mines. Structural hazards must also be considered as rudimentary facilities can be prone to collapse, a risk amplified by the use of explosives. Lastly, environmental and atmospheric hazards are plentiful. Smoke due to fire or gases discharged by weapons can reduce the quality of the air. This effect is further compounded by the need for ventilation in underground structures which may be damaged during fighting. The risk of explosions can be increased by gases and oxygen-rich air and the confined spaces can cause overpressure from blast waves (16).
Countermeasures
The countermeasures employed to combat tunnels are broadly twofold. On the one hand, military means previously mentioned include the bombardment of suspected tunnels (typically the entrances) from the air or with artillery. However, this method is largely ineffective at fully destroying underground structures. To truly destroy tunnels infantry and engineer units are often required to detect, infiltrate and destroy or deny the enemy access to the tunnel systems and subterranean structures. In that case, special considerations must be made for the hazards posed by tunnels. Additional equipment such as self-contained breathing apparatuses, night-vision devices, suppressors for weapons, and hearing protection may be required (17).
The other and more crucial countermeasure focuses on the detection of tunnels. Detecting tunnels can be as simple as spotting suspicious mounds of earth indicating excavation; however, a range of intelligence sources can also be used. Open-source intelligence can include geological surveys, cave surveys, photographs, or blueprints of urban structures. Intelligence may also be collected from human sources (HUMINT) or from intercepted insurgent transmissions (SIGINT). Technological means of detection consist of seismic sensing by use of acoustic waves, ground penetrating RADAR (GPR) which uses radio waves, or frequency domain electromagnetic mapping (FDEM) which assesses the electromagnetic conductivity of various materials. However, these technologies have shortcomings. High-frequency GPR is most effective up to 10 meters and lower-frequency seismic techniques lack the resolution to detect small structures such as tunnels, and FDEM is especially susceptible to interferences due to magnetic irregularities (18). These techniques are also heavily influenced by soil composition which may make them more or less effective depending on the environments.
Open-Source Intelligence & Field Examples
Footage released by the IDF showing troops uncovering subterranean structures in Gaza.
Footage originally published by Hamas showing militants emerging and retreating to a tunnel entrance after engaging an Israeli tank.
Endnotes & Works Cited (Chicago-style)
(1) - Olson and Morton, “Why Were the Soil Tunnels of Cu Chi and Iron Triangle in Vietnam So Resilient?”
(2) - Bahmanyar, “Afghanistan Cave Complexes.”
(3) - Kelso, “Ancient water trenches give Taliban ideal defences.”
(4) - Runkle, “Preparing for Warfare’s Subterranean Future.”
(5) - Ahronheim, “Hezbollah Has Inter-Regional Tunnel Network Stretching Hundreds of Km.”
(6) - Hernandez & Holder, “The Tunnels of Gaza.”
(7) - Richemond-Barak, Underground Warfare.
(8) - Runkle, “Preparing for Warfare’s Subterranean Future.”
(9) - Ibid.
(10) - Olson and Morton, “Why Were the Soil Tunnels of Cu Chi and Iron Triangle in Vietnam So Resilient?”
(11) - Runkle, “Preparing for Warfare’s Subterranean Future.”
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(12) - Ibid.
(13) - Department of the Army, “ATP 3-21.51 Subterranean Operations.”
(14) - Przystanek Historia “I was a sewer rat. Young Warsaw insurgents beneath the streets of the fighting capital.”
(15) - Spencer, “When War Goes Underground - Modern War Institute.”
(16) - Department of the Army, “ATP 3-21.51 Subterranean Operations.”
(17) - 9 Hole Reviews, “Fighting Underground - Sub-Terranean Military Operations in the Near Future.”
(18) - Slesinger, “A Cartography of the Unknowable: Technology, Territory and Subterranean Agencies in Israel’s Management of the Gaza Tunnels.”​​
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9 Hole Reviews. “Fighting Underground - Sub-Terranean Military Operations in the Near Future.” 9hpodcast.com, 2018. https://www.9hpodcast.com/e/fighting-underground-sub-terranean-military-operations-in-the-near-future-podcast-ep-7/.
Ahronheim, Anna. “Hezbollah Has Inter-Regional Tunnel Network Stretching Hundreds of Km.” The Jerusalem Post | JPost.Com, August 14, 2021. https://www.jpost.com/middle-east/hezbollah-has-inter-regional-tunnel-network-stretching-hundreds-of-km-676633.
Bahmanyar, Mir. Afghanistan Cave Complexes 1979–2004: Mountain strongholds of the Mujahideen, Taliban & Al Qaeda. Bloomsbury Publishing, 2012.
Department of the Army. “ATP 3-21.51 Subterranean Operations.” Department of the Army, 2016. https://armypubs.army.mil/epubs/DR_pubs/DR_a/pdf/web/ARN19656_ATP%203-21x51%20%20FINAL%20WEB.pdf
Hernandez, M., & Holder, J. “The tunnels of Gaza.” The New York Times, November 1, 2023. https://www.nytimes.com/interactive/2023/11/10/world/europe/hamas-gaza-tunnels.html
Kelso, Paul. “Ancient water trenches give Taliban ideal defences.” The Guardian, November 1, 2001. https://www.theguardian.com/world/2001/nov/02/afghanistan.terrorism2
Olson, Kenneth R., and Lois Wright Morton. “Why Were the Soil Tunnels of Cu Chi and Iron Triangle in Vietnam So Resilient?” Open Journal of Soil Science 07, no. 02 (January 1, 2017): 34–51. https://doi.org/10.4236/ojss.2017.72003.
Przystanek Historia. “I was a sewer rat. Young Warsaw insurgents beneath the streets of the fighting capital.,” October 26, 2012. https://przystanekhistoria.pl/pa2/tematy/english-content/43393,I-was-a-sewer-rat-Young-Warsaw-insurgents-beneath-the-streets-of-the-fighting-ca.html.
Richemond-Barak, Daphné. Underground Warfare. Oxford University Press, 2018.
Runkle, Benjamin. “Preparing for Warfare’s Subterranean Future - War on the Rocks.” War on the Rocks, August 10, 2015. https://warontherocks.com/2015/04/preparing-for-warfares-subterranean-future/.
Slesinger, Ian. “A Cartography of the Unknowable: Technology, Territory and Subterranean Agencies in Israel’s Management of the Gaza Tunnels.” Geopolitics 25, no. 1 (February 8, 2018): 17–42. https://doi.org/10.1080/14650045.2017.1399878.
Spencer, John. “When War Goes Underground - Modern War Institute.” Modern War Institute, April 17, 2020. https://mwi.westpoint.edu/war-goes-underground/.