Musée des Egouts

For centuries, the main source of drinking water in Paris was the Seine, which was also the main sewer. Construction of an underground sewerage system began at the time of Napoleon. Today, the Egouts de Paris constitutes a smelly museum; each sewer in the 2,100km (1,305-mile) system is marked with a replica of the street sign above. The Egouts can be closed after periods of heavy rain.