Tunnel network

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.

Tunnel networks were sometimes developed during siege warfare, even dating back to classical antiquity. Starting with a single tunnels being dug to undermine a wall that might be detected by the defenders and met with counter-tunnels, leading to tunnel warfare. Defenders might first create a series of underground listing posts to preempt such mining attacks.

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 underground tunnel network. It included underground barracks, shelters, ammo dumps and depots, and even had its own underground narrow gauge railways.

The tunnels of Củ Chi are an immense network of connecting underground tunnels located in the Củ Chi District of Ho Chi Minh City (Saigon), Vietnam, and are part of a much larger network of tunnels that underlie much of the country. The Củ Chi tunnels were the location of several military campaigns during the Vietnam War, and were the Viet Cong's base of operations for the Tết Offensive in 1968.
The tunnels were used by Viet Cong soldiers as hiding spots during combat, as well as serving as communication and supply routes, hospitals, food and weapon caches and living quarters for numerous North Vietnamese fighters. The tunnel systems were of great importance to the Viet Cong in their resistance to American forces, and helped to counter the growing American military effort.

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