Tunnel

A tunnel is an underground 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. Some tunnels are constructed purely for carrying water (for consumption, for hydroelectric purposes or as sewers); others carry other services such as telecommunications cables.

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).

A subway in the UK is a pedestrian tunnel that goes under a road.

The St. Gothard Tunnel opened in Switzerland on September 5, 1980 as the world's longest highway tunnel at 10.14 miles (16.32 km) stretching from Goschenen to Airolo.

Construction

Shallow tunnels are of the cut-and-cover type (if under water of the immersed-tube type), deep tunnels are excavated, often using a tunnelling shield. 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.

Wartime tunnels

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"?)

Examples of tunnels

The Lincoln Tunnel between New Jersey and New York is the busiest vehicular tunnel in the world.
The Sapperton Tunnel in the Thames & Severn Canal in England, dug through hills, which opened in 1789, was 3.5 km long and allowed ship transport of coal.
The Box Tunnel in England, which opened in 1841, is one of the oldest railway tunnels in the world. It is dug and has a length of 2900 m.
The Atlantic Avenue Tunnel, Brooklyn, New York is the world's oldest underground railway tunnel in a street, built in 1844 by the cut-and-cover method for the Long Island Rail Road. It is 800 m long.
The Channel tunnel between England and France.