Underground tunnels are passages that are built below the surface of the ground. They can serve various purposes, such as transportation, utilities, or storage. Many cities have subway systems that use tunnels to move people quickly and efficiently. Tunnels can also be used for water pipelines or sewer systems to manage essential services. These tunnels are constructed using different methods, depending on their purpose and location. Engineers often use tunneling machines to dig through soil and rock. Safety is a key concern, so tunnels are designed with proper ventilation and emergency exits to protect those who use them.