Tunneling refers to the process of excavating a hole in the ground or through a mountain to create an underground passageway. This type of construction is typically used for transportation systems, such as subways and trains, but can also be applied to other purposes like water supply pipelines or sewage lines. The tunnel is created by digging into the earth and then supporting the walls with various methods depending on the geology and location of the tunnel. Once completed, tunnels provide a safe and efficient way for people and goods to travel underground, avoiding obstacles such as rivers, highways or urban areas that would otherwise require detours.