The Musée des Égouts is a working sewer that visitors are allowed to walk through. Before the 1850s the city drank from the Seine downstream of its own waste, and cholera came back every few years. The engineer Eugène Belgrand was told to fix both ends of the problem at once. Clean water would come in from springs a hundred kilometres away by aqueduct, and everything else would go out through a separate network big enough for a man to stand up in. He dug one tunnel under each street.
The stretch you walk is a few hundred metres under the Left Bank quays, and the flow is real. It runs past you in an open channel with a walkway alongside. Blue enamel plates on the wall repeat the names and numbers of the streets overhead, so you can work out exactly whose building you are under. Cases along the route hold the tools the sewer workers used. One is a heavy ball that the current rolls down the pipe to scour it, another a gate on wheels that was dragged through to push silt along. It smells. Heavy rain can shorten the route or close it.