You can visit a little part of the Paris sewer system. The entrance is close to Pont d’Alma. You can read more about it here.
Actually what you find today underground is a combination of the sewer system and the double water supply network, drinking and non-drinking water. The same corridors and tunnels are used. All in all there are today 2.400 km (1.500 miles) of tunnels.
Walking around for about an hour, you will notice that the water leaving our buildings and streets is not quite clean. It’s good to know that it’s not going back to the Seine as such, but is brought far away and well cleaned before finally finding its way back to the river, far away downstream.
You will be taught the history of how Paris has taken care of its water supply and sewer system from ancient times until today and you can also see some of the tools in use, or previously used. The different corridors have the corresponding street names indicated.