A tunnel is a passage through a mountain or under a waterway, road or railroad.
It may be for pedestrians and/or cyclists, for general road traffic, for motor vehicles only, for rail traffic, or for a canal.
Various combinations are also possible.
The central part of a metro network is usually built in tunnels. To allow non-level crossings, some lines are in deeper tunnels than others. At metro stations there are often also pedestrian tunnels to walk from one platform to another.
At train stations of ground-level railways there are often one or more pedestrian tunnels under the railway to reach the platform(s).
Shallow tunnels are of the cut-and-cover type (if under water of the immersed-tube type), deep tunnels are excavated. For intermediate levels, both methods are possible.
Cut-and-cover is a method of tunnel construction where a trench is excavated and roofed over. Strong supporting beams are necessary to avoid the danger of the tunnel collapsing.
Castles, sappers
trench warfare: Crimea, US Civil War, WWI
Germany WWII, V2 factories, slave labor
North Korea, infiltrators, midget subs...
Japan, Corregidor, etc. (Neal Stephenson's Cryptonomicon?)
Vietnam, tunnel rats ("Platoon"?)
See also: List of tunnels, Wind tunnel.
