The Channel Tunnel is an undersea rail tunnel that links the United Kingdom and France. Opened in 1994, it carries both passenger and freight trains. The tunnel stretches 50.5 kilometres (31.4 miles) in length and is 75 metres (246 feet) deep at its lowest point.
The first ever proposal for a tunnel between the UK and France was put forward by a French mining engineer named Albert Mathieu back in 1802.
As well as being built for human use, tunnels can also be built for the safety and convenience of animals. More than 600 tunnels have been built under roads in the Netherlands to help increase the population numbers of endangered animals such as the European Badger.
Tunnel boring machines are often used to excavate major tunnels. These huge machines can bore through all kinds of sand, clay and hard rocks. The largest tunnel boring machines feature diameters of over 14 metres (46 feet).
When dealing with water crossings, building a tunnel is usually more expensive than building a bridge. Tunnels however preserve the above water scenery, are unaffected by weather conditions, and require less land on the shore.
The Lincoln Tunnel is a well known tunnel in the USA, it connects Weehawken, New Jersey and Manhattan, New York. Stretching 1.5 miles (2.4 kilometres) under the Hudson River, the Lincoln Tunnel has an average daily traffic of around 110000 vehicles.
At 137 kilometres (85 miles) in length, the Delaware Aqueduct in New York, USA, is the longest tunnel in the world.
The longest undersea tunnel in the world (as of 2010) is the Seikan Tunnel in Japan. Connecting the Japanese islands of Honshu and Hokkaido, the tunnel is 53.85 kilometres (33.46 miles) in length.
The Cu Chi tunnels found under the Cu Chi district of Ho Chi Minh City in Vietnam are a complex network of connecting tunnels that form part of an even larger network that spread throughout Vietnam. The tunnels were used frequently during the Vietnam War by Viet Cong to store food and weapons, hide, communicate, and provide sheltered areas for medical attention.
Aware of the Viet Cong’s strategic use of the tunnels, the US tried to destroy them. The tunnels however featured a range of booby traps and explosives that, combined with the difficulty in navigating the caves, made it too dangerous for the US to be very successful in their attempts to destroy the tunnels.