Tunnels are passageways that are built under the ground. They can also be made under a sea or a river. Some tunnels are dug through high mountains.
For hundreds of years, tunnels have been built all over the world. People use these tunnels to travel from one place to another. Some tunnels are used as waterlanes. Coal and gemstones are also brought from under the ground through long tunnels.
In the past, people dug tunnels using picks and shovels. Rocks and soil were loaded into one-wheel barrows and then onto horses and carts. This work was very tiring.
Today, engineers do a lot of planning before they start to build a tunnel. They use computers to check the level of the water and the types of rock under the ground. When they are sure the area is safe, huge machines are used to dig out the soil.
These machines have an enormous cutting wheel at the front. The wheel slowly grinds (磨碎) through the rock and soil to make the tunnel. Soil that is not needed is carried away in large trucks. As the tunnel becomes longer, it is lined with huge pipes so the walls and roof do not callapse.
Long tunnels have powerful lights to help drivers see the road ahead. Water sprinklers (洒水装置) are fitted to help keep people safe if a fire breaks out in the tunnel.
In large cities, tunnels are built under very busy roads. , People can drive through these tunnels. This makes it quicker to travel to different parts of the city.
Some trains have to wind slowly and carefully over steep mountain slopes. But, in places where there are tunnels, trains can travel quickly through the mountain to reach the other side.
