What is Tunnel?
{{no sources|date=December 2020}} {{main|Civil engineering}} [[File:Alter Elbtunnel - Ostroehre.JPG|260px|thumb|right|A tunnel for car movement in [[Hamburg]], [[Germany]].]] A '''tunnel''' is an underground passage. Some tunnels are used for [[car]]s, and others are used for [[Railway|trains]]. Sometimes, a tunnel is used for movement of [[ship]]s. Some tunnels are built for [[communication]] cables and some are built for [[electricity]] cables. Other tunnels are built for animals.<ref>[http://www.google.com/images?client=flock&channel=fds&q=tunnel&oe=utf-8&um=1&ie=UTF-8&source=univ&sa=X&ei=KGp5TeyKNM-OrQHP9cD9BQ&ved=0CEYQsAQ&biw=984&bih=514 Images for tunnels]</ref>[[File:Milford Tunnel.jpg|thumb|North portal of Milford railway tunnel, [[England]].]] {{Commonscat|Tunnels|tunnels}} Tunnels are dug in different kinds of [[ground]]s, from soft [[sand]] to hard [[Rock (geology)|rock]]. The way of digging is chosen by the type of ground. There are two additional ways of digging: [[quarry]] and 'cut and cover'. In quarry, the tunnel path is drilled in a horizontal way. This system requires a deep tunnel that's built in a firm rock. In the 'cut and cover' system, a tunnel is dug in the ground and then afterwards, a [[roof]] is built above the tunnel. This system fits tunnels that are close to the ground like road tunnels and [[infrastructure]]. Building tunnels is a large [[civil engineering]] project that could cost very high sums of money. The planning and building of a long tunnel may take many years. The [[Channel Tunnel]] between [[France]] and [[England]] is one of the longest tunnels in the world. It is 50 kilometers long. The longest tunnel in the world, the ''Gotthard Base Tunnel'', is being dug in [[Switzerland]]. == Reasons to build a tunnel == * A [[Rapid transit|subway]] is based on a network of tunnels that are dug underground so the trains will not disturb and will not be disturbed by the local transport. * On the path of a [[railway track|railroad track]] or a [[road]] a tunnel is dug when the lane