What is a tunnel?

A tunnel is an underground or underwater passageway that is usually constructed to connect two points or to provide a means of transportation or communication. Tunnels can vary in length, diameter, and shape, depending on their purpose and location. They can be used for a variety of purposes, from road and railway transport, to utilities, such as water and gas pipelines, and communication infrastructure, such as internet and phone cables. Tunnels can be accessed via vertical shafts, elevators, or inclined ramps, depending on their depth and location. They are usually constructed using various techniques, including drilling, blasting, and tunnel boring machines. Safety measures, such as ventilation and lighting, are implemented to ensure the safety of workers and users.