Underground systems refer to networks or structures located below the earth's surface. These can include tunnels, subways, and utility lines that transport water, electricity, and telecommunications. They are essential for urban infrastructure, helping to reduce surface congestion and protect utilities from weather-related damage. In cities, underground systems often include metro systems that facilitate public transportation, allowing commuters to travel efficiently. Additionally, sewer systems manage waste and prevent flooding, while mining operations extract valuable resources. Overall, these systems play a crucial role in modern society by enhancing connectivity and supporting essential services.