While Paris, the City of Lights, shines brightly, hidden in its shadows are fascinating installations, put in place nearly two centuries ago: the sewers. Even today, this 2,600-kilometer-long labyrinth of streets and infrastructure ensures the sanitation of the entire capital. In 1854, during Haussmann's grand urban renewal projects in Paris, the French engineer Eugène Belgrand undertook one of the century's largest construction projects: the renovation of the Paris sewers. His main innovation: an ingenious dual network that simultaneously treats wastewater and delivers drinking water to Parisians.