Please keep me anonymous. I have sort of a respectable job, and I'd rather not have someone come across my past adventures...

There were at least two, and possibly three sets of tunnels that came together at the Power Plant. One was the set under the Diag and the other was the set that connected Markley and the various dorms in that area with the Med School complex. I heard there was another set that fed the northwest edge of the campus near the Rackham building, which I never set foot in. The major highlight, I suppose, were the fetuses in jars in a storeroom of the Buhl Medical Science Building at the Med School accessed via a part of the tunnels known "Hole in the Wall". I never tried to see if the tunnels under the Diag connected to those feeding the hill dorms & Med School because I was too paranoid to go near the Power Plant, especially from the Med School system, because it was a long straight and usually well-lit (and very exposed) passageway leading to the power plant (and I wanted to graduate without a police record). I imagine there are nasty security devices down there now.

Places I used to get in or out (remember, these are 21 year-old recollections -- things may have changed):

Central Campus

1) Grating in front of the Dental School.
2) CC Little Building - easy out, hard to find in.
3) The Law School - any of the doors at the entrance to each dorm, under each archway leading into the law quad (or the door described above).
4) Either of the two stairs leading down from the lowest floor of the Nat Sci Building (great way out, locked doors always prevented entry)
5) Trapdoor in broom closet on first floor of Michigan Daily offices. (A great concept.)
6) Basement of Martha Cook residence (they used to store bikes etc. there)
7) Various doors in West Quad.
8) I never explored around East Quad, so I don't know if there is an entrance there. However, my tunnelling career began when two guys in tunnel garb from East Quad popped their heads out of a wall on the loading dock of Markley (see below), and asked me where they were. A whole new world opened up before me.... (The only thing is that to get from East Quad to Markley, they would have had to go through or perilously close to the Power Plant.)
9) The basic layout, visible with snow on the ground, is a big loop around the very center of campus with various offshoots, such as across South University between the Art Museum and the Law Quad.
10) There was a rumor of an old swimming pool under the Michigan Union. The tunnels go into the sub-basement of the Union, and that in itself looked like an interesting area to explore, but I never did.

Hill/Medical Complex

1) Stairway in Boiler Room off the loading dock of Markley Hall. There was a locked door at the bottom of the stair, but a convenient hole was punched into the wall next to the doorknob. At the time, the Boiler Room could be conveniently entered through a hole created by 4 missing cinderblocks above the door!
2) The Victor Vaughan Building
3) Fetus Storeroom in Buhl Medical Building (Hole in the Wall)
4) I never found a way in/out of Alice Lloyd, though the tunnels put you into the foundation. I can't remember if there was a door in or out of Mo-Jo or Couzens. There was a small door leading into Stockwell, but it was always locked.

Rackham

1) Total mystery to me. I never set foot in those tunnels, if they exist.