An extensive tunnel system exists beneath the city of Paris, playing a major infrastructural role, but also serving as an interesting tourist attraction and look at the underbelly of the historic city.
"An extensive network of abandoned quarries, sewers and subway lines twists beneath modern Paris."
"By 1813, the year digging beneath Paris was banned to prevent further destabilization of the ground, some 170 miles of labyrinthine tunnels had been carved far below the city proper. In 1786, to stanch the spread of disease from overcrowded cemeteries, a portion of these old quarries were consecrated as burial grounds, and human remains were transferred there. Burials in the newly anointed 'catacombs,' both direct and as cemetery transfers, continued until 1860."
"Napoléon Bonaparte ordered the creation of the underground sewer system, now some 300 miles long, in the early 19th century. Baron Georges-Eugène Haussman, the urban planner who shaped modern Paris, expanded the network, and it was finally completed in 1894 under Napoleon III."