Tunnel

The Shanhe tunnel boring machine bored a tunnel under the Yellow River.

In ordinary usage, a tunnel is an artificial underground connection between two or more points on the surface. The term is also used in particle physics.

Underground tunnels are used to carry trains, highways, water pipes, telecommunication cables, pedestrians.

Some tunnels were built solely for stealth. Some fortresses were built with stealth tunnels, to enable occupants to circumvent a seige, or bring in supplies, during a seige. Alternately, some beseigers dug tunnels so they could pop up unexpectedly, inside a beseiged fortress. Beseigers might build tunnels intended to undermine the foundations of a fortress wall, or to plant a cache of explosives under the fortress large enough to cause major damage.

In ancient times tunnels were dug using brute force manual labor. The development of early explosives, like gunpowder, could be used to ease the burden of tunneling through rock.

In recent centuries tunneling relies on powerful machinery, particularly tunnel boring machines.