A tunnel is an underground passage that may be used for car or boat traffic, subways, pedestrians, water or sewage, or power and communication line. Some of the earliest tunnels were the roman aqueducts. They were used to carry clean water into the cities and transport sewage away. Even without electric machinery, the Romans constructed tunnels that lasted until modern time.
The method used to create a tunnel depends on where it is to be constructed and what material will surround it. A tunnel through mountain rock is usually made with a tunnel-boring machine, or mole. This powerful machine is shaped like a large cylinder: it cuts and grinds through rock, leaving behind a passageway. Because rock surrounds the tunnel, extra support isnâ€™t necessary during construction. Explosives were used before the invention of moles, but moles are safer and quieter. They also leave behind a smoother tunnel of the exact shape the engineers want.