Pretty interesting to see the people living in the tunnels underneath Vegas. http://www.channelapa.com/2009/09/tunnel-people-in-vegas-by-lisa-ling.html
How does it not smell down there? Tunnel is a nice word for sewer. It carries the runoff from the strip. While it's not raw sewage, I imagine it's pretty nasty.
These are actually redirections of the natural washes that occur throughout the Vegas valley. These are, for most of the year, dry. The problems occur whenever a flash flood occurs and the tunnel residents do not have enough time to clear out...plus they usually end up losing most of their accumulated items during a flood.