Tunnel

A tunnel is an underground passage. Some tunnels are used for cars, and others are used for trains. Sometimes, a tunnel is used for movement of ships. Some tunnels are built for communication cables and some are built for electricity cables. Other tunnels are built for animals.

The Channel Tunnel between France and England is one of the longest tunnels in the world. It is 50 kilometers long. The longest tunnel in the world, the Gotthard Base Tunnel, is being dug in Switzerland.

Some reasons to build a tunnel

 * A subway is based on a network of tunnels that are dug underground so the trains will not disturb and will not be disturbed by the local transport.
 * On the path of a railroad track or a road, a tunnel is dug when the lane encounters an obstacle such as a mountain to avoid bypassing the obstacle.
 * A tunnel is built sometimes to overcome a water obstacle as a replacement for building a bridge structure above it.
 * A tunnel is built to connect between military posts so the movement between them will not be visible for the enemy.
 * A tunnel is built for infrastructure like electricity cables, water, communication and sewerage to avoid damage and disruption above ground.