I'm hoping to revive a truly great thread, so I'm posting some draining pictures here.
My brother and I found this RCB tunnel somewhere in the greater Los Angeles area; it's a part of a storm drain system that I frequent.
Here's the entrance, a water-slide that's very slippery and somewhat difficult to climb.
Looking down the slide from the top...
Looking onward, the light from a manhole can be seen in the distance.
Looking up at one of several manholes...
Random locks on one of the rungs...
And some minor formations...
Further in the RCB gets smaller and a second path (sidepipe) is visible.
We stopped at this junction to do some light painting with sparklers.
Lastly, here's a picture of me on the way out.
Thanks for looking and I hope to see more explores from the LA draining community soon!