The term 'Underground Drainage System' refers to a system of pipes and tunnels that are used to transport wastewater away from buildings and other structures. The most common type of underground drainage system is the SEWER, which is a network of pipes that collect wastewater from homes, businesses, and other sources and transport it to a treatment plant. Sewers are typically made of concrete, clay, or plastic and are designed to be durable and long-lasting.