[[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]]
TunnelsA are'''tunnel''' undergroundis constructionsan underground usedor forundersea transportationspassageway. 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]] differs significantly from a tunnel,<ref>{{Cite journal |last=Zhou |first=Man |last2=Su |first2=Xiaolong |last3=Chen |first3=Yaying |last4=An |first4=Lin |date=2022-10-02 |title=New Technologies and Challenges in the Construction of the Immersed Tube Tunnel of the Hong Kong-Zhuhai-Macao Link |url=https://www.tandfonline.com/doi/full/10.1080/10168664.2021.1904487 |journal=Structural Engineering International |language=en |volume=32 |issue=4 |pages=455–464 |doi=10.1080/10168664.2021.1904487 |issn=1016-8664}}</ref> though some recent tunnels have used [[immersed tube]] construction techniques rather than traditional tunnel boring methods.<ref>{{Cite book |last=Takahashi |first=Yutaka |title=Water Storage, Transport, and Distribution |publisher=[[EOLSS Publications]] |year=2009 |isbn=9781848261761 |pages=318-319 |language=EN}}</ref>
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>
