The current sewage system in London was built in the Victorian era, and is no longer adequate to cope with the demands of a modern city. When heavy rain falls, the sewers can become overwhelmed, and untreated sewage can be discharged into the River Thames, causing pollution that harms the environment and public health. The Thames Tideway Tunnel is designed to address this problem by providing extra capacity to store and treat the excess sewage during heavy rainfall, preventing it from overflowing into the river.