Constructing a tunnel can be one of the most complex challenges in the field of civil engineering. A tunnel is a passageway underground which can carry a road, pedestrians, trains, sewage, clean water, a river or canal, power cables or gas. There are three main types of tunnel: cut and cover tunnels; bored tunnels and immersed tube tunnels.
The way tunnel engineers design and construct a tunnel depends on the type of soil or rock under which the tunnel needs to be built and whether the tunnel needs to go under water. Sometimes tunnel engineers use explosives to blast through solid rock; sometimes they use enormous tunnel boring machines; sometimes a concrete tube is built on dry land and then floated into position before being sunk to the bottom of a river.