A tunnel is an underground passage. When designed for use by traffic, it may be called an underpass.
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), while others carry other services such as telecommunications cables.
Tunnels are dug in various types of materials, from soft clays to hard rocks. Depending on the type of soil, a method of excavation is chosen. When digging soft clays, the New Austrian Tunnelling method or NATM is used. When digging in weak rocks a tunnel boring machine or TBM is used. In hard rocks blasting is usually the fastest method, as in the Norwegian tunnelling method.
Various combinations of these methods 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.
Ground-level railway stationss often have one or more pedestrian tunnels under the railway to enable passengers to reach the platforms without having to walk across the tracks.
In the UK a pedestrian tunnel or other underpass beneath a road is called a "subway".
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.
Tunnel boring machines can be used to automate the entire tunneling process.
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.
"Tunnel" was also the name of a large New York City nightclub with multiple rooms themed after subway tunnels. It was perhaps best known for hosting Kurfew, its gay-oriented Saturday night parties with resident DJ Johnny Vicious. Tunnel closed its doors soon after the demise of nearby Twilo, the victim of the owner's debts and of Rudy Giuliani's quality-of-life campaign.
