By the RR tracks
Dani and I were going to hit the tunnel under the tracks by Vickery St. that we had only partially explored before.  Unfortunately, we could see from the road nearby that the same homeless people who occupied the area before were still there and were quite permanently encamped (they were living out of a tent but had a a clothes line for crying out loud!!!), so we were a little leery of being cornered in a tunnel.

As a result, we drove around the area to find out what was upstream.  Keen-eyed Dani spotted the entrance to a tunnel a ways off the road, so we checked it out instead.

Here's a shot of the ravine there the tunnels empty into.  The bridge in the background is where we spotted this site from.

You know, this would make a good location for a David Lynch project.   Just a thought.
This is the entrance to the tunnels.  Initially I thought it was the usual double-barrel parallel design, but they turned out to follow  two distinct courses.
Most of this tunnel was like this, 6 ft ceiling, with very few side tunnels to feed in from either side.  The few of these we saw were only about 12 inches in diameter.

Remarkably, most of the tunnel was this dry, with only one spot that was unexpectedly deep (about 3 or 4 inches) and muddy.
We went around a corner after just a few hundred feet (maybe a quarter of a mile total), and hit what was effectively a dead end.  There is a small tunnel at the end that Dani is looking up...
...which I climbed up into, only to discover a tiny room, maybe 4 to 5 ft high with a manhole at the top and two very small tunnels feeding into it.
So we doubled back and checked out the right side.

The flow coming through here was much stronger, which wasn't surprising given that there had been rain the previous night and off and on for the last few days.
As you can see ahead in the distance, this tunnel switches from the modern square configuration to the more classical archway style.
On to the next part