page_title: Tunnel network
section_title: Trench systems
hierarchical_section_title: Tunnel network / In warfare / Trench systems
caption_attribution_description: Gros Ouvrage du Michelsberg - Hohlgang
context_page_description: In transport, tunnels can be connected together to form a tunnel network. These can be used in mining to reach ore below ground, in cities for underground rapid transit systems, in sewer systems, in warfare to avoid enemy detection or attacks, as maintenance access routes beneath sites with high ground-traffic such as airports and amusement parks, or to extend public living areas or commercial access while avoiding outdoor weather.
context_section_description: Any time the use of trenches becomes extensive, this naturally leads to connecting them with tunnel networks for safe passage both along the trench lines and with rear areas. In World War I, when given enough time and resources, the underground components of the defenses could become more extensive than those above ground.
  The French Maginot Line, constructed from 1929 to 1939, was a chain of fortresses, bunkers, retractable turrets, outposts, obstacles, and sunken artillery emplacements, all linked by an extensive shell-proof tunnel network. It included underground barracks, shelters, ammo dumps and depots, and even had its own underground narrow gauge railways.
caption: Connecting tunnel under Ouvrage Michelsberg, a fortress of the Maginot Line
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