Musée des égouts de Paris.
Tour a chunk of the Paris sewer system, which has 1,300 miles of human-accessible tunnels.

They clean the silt out of some of the drainage tunnels by sending a huge ball down them, kind of like the one in "Raiders of the Lost Ark."

Related links:
-- Paris sewer tour by Peter van der Linden
-- "Waste matters on Paris tours," Baltimore Sun
-- Historical engravings and photos from sewerhistory.org

For further research:
-- Paris-related links from Undercity.org
-- Zone-Tour: Database of Urban Exploration

I am seeking volunteers for a collaborative project. If you have access to a digital camera or other device that can make a primitive little video clip, and you want to participate, send an e-mail to project at lightningfield dot com.