Tunnel warfare
Israel-Gaza Conflict: Sometimes the ongoing conflict between the Israeli Army and Islamist militants in Hamas-governed Gaza Strip is called a tunnel conflict.
In 2017, the Barrier against tunnels along the Israel-Gaza Strip border began to be built to prevent the digging of cross-border attack tunnels. On October 30 such tunnel was located within Israeli borders and was detonated.
Hamas has constructed an extensive network of tunnels under Gaza City and other populated areas of the Gaza Strip, sometimes called the Gaza metro. The network is over 500 km (300 mi) in length according to Hamas claims. According to experts, these tunnels serve multiple purposes for Hamas, including holding kidnapped hostages, smuggling goods, moving militants, storing weapons, and sheltering Hamas members and infrastructure. However, the location and use of military tunnels in densely populated areas have raised concerns about Hamas endangering civilians.
It has been reported during the ongoing 2023 Israel-Hamas war that Hamas has dug extremely extensive tunnels under Gaza, and that capturing and destroying the tunnels is a "top priority" of the IDF. A 2024 Royal United Services Institute report details Hamas's use of two types of tunnels: deep, well-equipped ones for high-ranking commanders and shallower ones for lower-level members. Initially, the Israeli Defense Forces (IDF) planned to secure territory before searching for tunnels, but this strategy allowed Hamas to launch ambushes from underground. This experience highlighted the need for effective counter-tunnel operations to combine both surface and subterranean combat, while also addressing the risk of friendly fire. See Subterranean warfare#2023 Israel–Hamas war for more on the subject.

Russian invasion of Ukraine: In 2022, Ukrainian resistance managed to hold off the Russian invading army for 80 days by using tunnels underneath the city of Mariupol. By April 2022, Russian and separatist troops had pushed deep into most of the city, separating the last Ukrainian troops from the few pockets of Ukrainian troops retreating into the Azovstal Iron and Steel Works, which contains a complex of bunkers and tunnels which could even resist a nuclear bombing.

See also: Attrition warfare
Breastwork (fortification)
Early thermal weapons
Explosive mine
Land mine
Maneuver warfare
Sapper
Sapping
Siege warfare
Subterranean warfare
Trench warfare

Citations: 

General references: Ebrey, Walthall, Palais (2006). East Asia: A Cultural, Social, and Political History. Boston: Houghton Mifflin Company.
Ernst, O. H. (1873). A Manual Of Practical Military Engineering, Prepared for the Use of the Cadets of the U. S. Military Academy, and for Engineer Troops. New York: D. Van Nostrand. Retrieved 2007-11-19.
Ulmer, D. S. (2008). Shaping Operational Design: A Counter to the Growing Trend of Underground Facilities and Tunnel Warfare. Air Command and Staff College.
Zhang, Song Shan (张嵩山) (2010), Decipher Shangganling (解密上甘岭) (in Chinese), Beijing: Beijing Publishing House, ISBN 978-7-200-08113-8

Further reading: Jones, Simon (2010). Underground Warfare 1914–1918. Pen & Sword Military. ISBN 978-1-84415-962-8.

External links: 
Vietnam war tunnels at warchapter.com Archived 2012-12-27 at the Wayback Machine
Vietnamese source on tunnel warfare
Modern US Army source on tunnel warfare
YouTube video of Hill 60 on the Messines Ridge, site of a huge underground explosion