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[[File:Southern portal of Chirk canal tunnel - geograph.org.uk - 1293092.jpg|thumb|Southern portal of the {{convert|421|m|ft|abbr=on|adj=mid|long}} [[Chirk Tunnel|Chirk canal tunnel]], Wales]]
A '''tunnel''' is an underground or undersea passageway. It is dug through surrounding soil, earth or rock, or laid under water, and is usually completely enclosed except for the two [[Portal (architecture)|portals]] common at each end, though there may be access and ventilation openings at various points along the length. A [[Pipeline transport|pipeline]]
A tunnel may be for foot or vehicular [[road]] [[traffic]], for [[rail transport|rail]] traffic, or for a [[canal]]. The central portions of a [[rapid transit]] network are usually in the tunnel. Some tunnels are used as [[sanitary sewer|sewers]] or [[aqueduct (watercourse)|aqueducts]] to supply water for consumption or for [[hydroelectric]] stations. [[Utility tunnel]]s are used for routing steam, chilled water, electrical power or telecommunication cables, as well as connecting buildings for convenient passage of people and equipment.<ref>Salazar, Waneta. ''Tunnels in Civil Engineering''. Delhi, India : White Word Publications, 2012.</ref>
[[Secret passage|Secret tunnels]] are built for military purposes, or by civilians for [[smuggling]] of [[weapon]]s, [[contraband]], or [[people]].<ref>{{Cite journal |last=Sorrensen |first=Cynthia |date=2014-07-01 |title=Making the Subterranean Visible: Security, Tunnels, and the United States–Mexico Border |url=https://www.tandfonline.com/doi/full/10.1111/j.1931-0846.2014.12029.x |journal=Geographical Review |language=en |volume=104 |issue=3 |pages=328–345 |doi=10.1111/j.1931-0846.2014.12029.x |issn=0016-7428}}</ref> Special tunnels, such as [[wildlife crossing]]s, are built to allow wildlife to cross human-made barriers safely.<ref>Brodziewska, J. (2005). Wildlife tunnels and fauna bridges in Poland: past, present and future, 1997-2013. ''UC Davis: Road Ecology Center''. Retrieved from <nowiki>https://escholarship.org/uc/item/4wd0j27j</nowiki></ref> Tunnels can be connected together in [[tunnel network]]s.
== Terminology ==▼
[[File:Tampereen rantatunneli.jpg|thumb|An entrance of the [[Rantaväylä Tunnel]] in the northern part of [[Tampere]], [[Pirkanmaa]], [[Finland]]]]▼
[[File:Scene with Fabric Tunnel - Srimangal - Sylhet Division - Bangladesh (12904860703).jpg|thumb|A fabric tunnel in [[Moulvibazar District]], [[Bangladesh]]]]▼
A tunnel is relatively long and narrow; the length is often much greater than twice the [[diameter]], although similar shorter excavations can be constructed, such as cross passages between tunnels.
The definition of what constitutes a tunnel can vary widely from source to source. For example, in the United Kingdom, a road tunnel is defined as "a subsurface highway structure enclosed for a length of {{convert|150|m|ft}} or more."<ref>{{cite book |url=https://www.standardsforhighways.co.uk/tses/attachments/987a669b-13a1-40b9-94da-1ea4e4604fdd |title=Highway Structures & Bridges Design CD 352, Design of road tunnels (formerly BD 78/99) |publisher=The Department for Transport |year=2020}}</ref> In the United States, the [[National Fire Protection Association|NFPA]] definition of a tunnel is "An underground structure with a design length greater than {{convert|23|m|ft|abbr=on}} and a diameter greater than {{convert|1800|mm|ft}}."<ref>{{cite book |title=NFPA Standard for Safeguarding Construction, Alteration, and Demolition Operations |publisher=National Fire Protection Association}}</ref>▼
▲== Terminology ==
▲{{Empty section|date=September 2024}}[[File:Tampereen rantatunneli.jpg|thumb|An entrance of the [[Rantaväylä Tunnel]] in the northern part of [[Tampere]], [[Pirkanmaa]], [[Finland]]]]
▲[[File:Scene with Fabric Tunnel - Srimangal - Sylhet Division - Bangladesh (12904860703).jpg|thumb|A fabric tunnel in [[Moulvibazar District]], [[Bangladesh]]]]
== Etymology ==
▲The definition of what constitutes a tunnel can vary widely from source to source. For example, in the United Kingdom, a road tunnel is defined as "a subsurface highway structure enclosed for a length of {{convert|150|m|ft}} or more."<ref>{{cite book |url=https://www.standardsforhighways.co.uk/tses/attachments/987a669b-13a1-40b9-94da-1ea4e4604fdd |title=Highway Structures & Bridges Design CD 352, Design of road tunnels (formerly BD 78/99) |publisher=The Department for Transport |year=2020}}</ref> In the United States, the [[National Fire Protection Association|NFPA]] definition of a tunnel is "An underground structure with a design length greater than {{convert|23|m|ft|abbr=on}} and a diameter greater than {{convert|1800|mm|ft}}."<ref>{{cite book|title=NFPA Standard for Safeguarding Construction, Alteration, and Demolition Operations|publisher=National Fire Protection Association}}</ref>
The word "tunnel" comes from the [[Middle English]] ''tonnelle'', meaning "a net," derived from [[Old French]] ''tonnel'', a diminutive of ''tonne'' ("cask"). The modern meaning, referring to an underground passageway, evolved in the 16th century as a metaphor for a narrow, confined space like the inside of a cask.<ref>{{Citation |title=tunnel, n. |date=2023-03-02 |work=Oxford English Dictionary |url=https://oed.com/dictionary/tunnel_n |access-date=2024-09-01 |edition=3 |publisher=Oxford University Press |language=en |doi=10.1093/oed/1137765320}}</ref> The modern sense of it as a passageway emerged as engineering and mining practices grew.
== History ==
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Some of the earliest tunnels used by humans were [[Paleoburrow|paleoburrows]] excavated by prehistoric mammals.<ref>{{Cite journal |last=Frank, de Carvalho Buchmann, Gonçalves de Lima, Caron, Lopes & Fornari |date=2011 |title=KARSTIC FEATURES GENERATED FROM LARGE PALAEOVERTEBRATE TUNNELS IN SOUTHERN BRAZIL |url=https://www.ufrgs.br/paleotocas/Frank_et_al_2011b.pdf |journal=Espeleo-Tema |volume=22 |issue=1}}</ref>[[File:Joralemon Street Tunnel postcard, 1913.jpg|thumb|Joralemon Street Tunnel on 1913 postcard, part of the [[New York City Subway]] system]]
Much of the early technology of tunneling evolved from [[mining]] and [[military engineering]]. The
=== Antiquity and early middle ages ===
Predecessors of modern tunnels were [[adit]]s that transported water for [[irrigation]], drinking, or [[sewerage]]. The first [[qanat]]s are known from before 2000
The earliest tunnel known to have been excavated from both ends is the [[Siloam tunnel|Siloam Tunnel]], built in [[Jerusalem]] by the kings of [[Kingdom of Judah|Judah]] around the 8th century BC. Another tunnel excavated from both ends, maybe the second known, is the [[Tunnel of Eupalinos]], which is a tunnel [[Aqueduct (water supply)|aqueduct]] {{convert|1036|m|ft|abbr=on|adj=on|sigfig=3}} long running through Mount Kastro in [[Samos Island|Samos]], [[Greece]]. It was built in the 6th century BC to serve as an [[Aqueduct (water supply)|aqueduct]].
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