thumb|A disused railway tunnel, near Houyet, Belgium, now converted to pedestrian and bicycle use File:LightTunnelDetroit.jpg|thumb|Colorful pedestrian Light Tunnel connecting two terminals in Detroit's DTW airport. thumb|The North East MRT Line in Singapore is a fully-underground rail line. thumb|The Allegheny Mountain Tunnel on the Pennsylvania Turnpike. thumb|A tunnel under the A1086 road carrying a burn and a footpath with a dam to protect the tunnel from being blocked by logs during storms, in Castle Eden Dene near Peterlee, England

A tunnel is an underground passageway. 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. Some civic planners define a tunnel as in length or longer, while anything shorter than this should be called an underpass or a chute. For example, the underpass beneath Yahata Station in Kitakyushu, Japan is only long and therefore should not be considered a tunnel.

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. Some tunnels are not for transport at all but are fortifications, for example Mittelwerk and Cheyenne Mountain.

In the United Kingdom a pedestrian tunnel or other underpass beneath a road is called a subway. This term was used in the past in the United States, but now refers to underground rapid transit systems.

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. Rail stations with much traffic usually provide pedestrian tunnels from one platform to another, though others use bridges.

Geotechnical investigation

main: Geotechnical investigation

It is essential that any tunnel project starts with a comprehensive investigation of ground conditions. The results of the investigation will allow proper choice of machinery and methods for excavation and ground support, and will reduce the risk of encountering unforeseen ground conditions. In the early stages, the horizontal and vertical alignment will be optimized to make use of the best ground and water conditions.