From here some stairs went back down on one side to a door, which led into some small intra-building tunnels. One direction ended at a door into one part of the basement, and the other direction continues to some branches and mech rooms. In some places the tunnel became a vertical shaft with multiple "tunnels" at the level of each floor and separated by catwalks. I've seen this in other buildings but they usually don't connect to anything else.
It's kind of hard to explain the multi-level thing, just look at the
fourth picture and think of a series of catwalks like the one above that
tunnel, each one serving as access to this pipechase canyon from each floor
of the building. The tunnels connected all these canyon areas (at least
two that I saw) to eachother at the lowest level of the building.
The tunnels also connected to some public hallways in various basement areas, and at least one mech room marked "Biohazard". Near this was a small hatch in the wall, I opened it to find the bottom of a dumbwaiter shaft.
The biohazard door and the room beyond, full of shiny steel tanks of...
something... The dumbwaiter shaft hatch is visible in the lower right
of the third picture, they forgot to lock it up but there was a warning
that opening the door didn't disable the dumbwaiter from moving.
On the way back to the mech room I heard a falling-water sound that I hadn't noticed before, and saw a manhole hidden under some pipes to one side of the tunnel. I pulled the lid off to find a deep sewer shaft going down towards the level of the real steam tunnels, I could barely make out what looked like a small sewer tunnel at the bottom (too small for a person), but I sure as hell wasn't going down there without backup. I'd mentioned that I was going tunneling and vaugely mentioned the building to one of my friends, but other than that no one knew where I was.
Ninja Turtles style! I'm pretty sure this is non human-passable at
the bottom, it goes back down towards the real tunnels but I don't recall
any connection other than occasional little sewer pipes.
After that I went into the main part of the building, took pictures of some floor plans to make a map, tried to get on the roof, and went home (I explored a construction site on the way but that's another update).
Top of that dumbwaiter, and a hallway as seen through a hole in a door.