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.

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.

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 only direct hit was from a bomb that failed to explode; the resulting hole was utilized as a ventilation shaft.

The terrorist group avoids battlefield engagements, preferring to hide in such tunnels safe from satellite detection, drone strikes and artillery, managed to maintain supply lines and communication with other areas under their control.