A tunnel is an underground passage. The definition of what constitutes a tunnel is not universally agreed upon. However, in general tunnels are at least twice as long as they are wide. In addition, they should be completely enclosed on all sides, save for the openings at each end.
A tunnel may be for pedestrians or cyclists, for general road traffic, for motor vehicles only, for rail traffic, or for a canal. Some are aqueducts, constructed purely for carrying water — for consumption, for hydroelectric purposes or as sewers — while others carry other services such as telecommunications cables. There are even tunnels designed as wildlife crossings for European badgers and other endangered species. Some secret tunnels have also been made as a method of entrance or escape from an area, such as the Cu Chi Tunnels or the tunnels connecting the Gaza Strip to Egypt.
In the United Kingdom a pedestrian tunnel or other underpass beneath a road is called a subway. This term was also used in the past in the United States, but is now used to refer to underground rapid transit systems.
In the Czech republic, the verb to 'tunnel'is a synonym for to embezzle. For example:the manager 'tunnelled' the company and now lives on the Bahamas; or, many banks collapsed because they were 'tunnelled'.
The central part of a rapid transit network is usually built in tunnels. To allow non-level crossings, some lines run in deeper tunnels than others. At metro stations there are usually pedestrian tunnels from one platform to another. Often, ground-level railway stations also have one or more pedestrian tunnels under the railway to enable passengers to reach the platforms without having to walk across the tracks. However, in the United Kingdom bridges are an equally popular method of pedestrian access between two or more different railway station platforms.