Well, I’ve been inside a large drainage pipe in San Clemente, CA. It wasn’t technically a sewer as it wasn’t carrying waste (it only had a small trickle of water running through it when I went in there), but my friends and I walked through it (it was big enough so you could stand, but you had to bend down slightly). Eventually, we got far enough into it that the light from the entrance was completely gone so that it was absolute pitch black inside (that was very odd) and my friends, being who they are, sparked up a J in the center of the tunnel. I was relieved to see the light at the other end, though… :P