Tunnels are mostly build for humans to make the distance to travel shorter or easier, but there are also places where people build special tunnels for animals to get to the other side of a high way or busy road. In Netherlands alone, there are more than 600 underground tunnels build to increase the safety of the animals, especially the endangered types like the European Badger.
The United Kingdom and France are connected via the Channel Tunnel and this carries both passenger and freight trains under the sea. It is 50.5 km long and at some places 75 km deep. The tunnel opened in 1994 even though there were earlier proposals for it to be built - can you imagine how much planning it took to build this tunnel? Do extra research and find a video on this.
With modern technology, it has become easier to dig a tunnel - the tunnel bore machine will be used to excavate sand and rock from a mountain. It can bore through different types of soil and rock.
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.
A well known tunnel that goes under water is the Lincoln Tunnel that connect New Jersey and Manhattan. It goes 2.4 km under the Hudson River and it carries more than 110 000 cars per day.
One of the longest tunnels in the world is the Delaware Aqueduct in New York which is 137 km long. There are many tunnels that carries cars and trains and water and are very long. You can have a look at a list of tunnels here.
