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A trip through a Metro System tunnel in GermanyA trip through a Metro System tunnel in GermanyClick to play video A tunnel is an underground passageway which is dug, blasted or bored out of the ground. A tunnel is needed when a road, railway or canal has to pass under an obstacle such as a river or mountain. The first tunnels were built in ancient times for aqueducts, channels to carry water from springs to supply cities. In some cities, such as London, Paris and New York, whole rail networks are built underground in a network of tunnels to prevent congestion at street level. Sewers are built as an underground network of tunnels and pipes.

Eupalinian tunnel, SamosEupalinian tunnel, Samos

Ancient tunnels

The Babylonians, Greeks and Romans all built tunnels in ancient times. In the 6th century BC, the Greek engineer Eupalinos built a tunnel just over a kilometre long on the island of Samos for an aqueduct, a channel to supply the capital city with fresh water. It was designed prevented the water supply being easily cut off by an enemy.

One of the first tunnels was built under the River Euphrates in the Babylon. In around 2160 BC, builders diverted the river and constructed a brick tube in a trench on the river bed. The finished tunnel, 900 m (3000 ft) long, allowed people to walk from the palace to the temple.

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