the first time I ever entered this drain, it was a cold rainy day. The water level was far too high to navigate this tunnel without ending up with some very cold very wet feet. So we came back one night when it was a little dryer, and found that despite the absolute blackness of this tunnel (no lights anywhere, no lights coming in from the manholes above), the tunnel was not without color. The bottom of the tunnel floor was thick with a golden brown sediment, perhaps some type of soil that had run off into this drain from some of the pipes and smaller drains that feed into this very large (and very smelly) drain.