tunnels and underground excavations - Encyclopedia Britannica
https://www.britannica.com/technology/tunnel
WEBtunnels and underground excavations, horizontal underground passageway produced by excavation or occasionally by nature’s action in dissolving a soluble rock, such as limestone. A vertical opening is usually called a shaft. Tunnels have many uses: for mining ores, for transportation —including road vehicles, trains, subways, and canals ...
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