Please keep me anonymous. I have sort of a respectable job, and I'd rather not have someone come across my past adventures...
I used to explore the tunnels a lot when I was an undergraduate at Michigan back in 1971-1975. We usually used to enter via the law school, through a flimsy padlocked door near the men's bathroom in the basement of the Law Club, Hutchins Hall (?) on the State Street edge of the Law Quad. We used to use the tunnels to get onto the basketball courts at the old Waterman gymnasium before it was torn down. (The construction site of the then new IM building was a great place to easily enter the tunnels during my senior year, because it was completely open and unlocked. We got written up in the Michigan Daily when we turned off the President's hot water from underneath his house -- we actually tried to inform the Daily by tunneling into their building via a trapdoor in its first-floor broom closet, but the door out of the closet was locked from the outside. When you head north from the law school up toward the LS&A building, and there are very old sections, such as at the north end of the Diag near the Chem and Nat Sci Buildings.
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):
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 tunneling 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.
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 cinder blocks 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.
1) Total mystery to me. I never set foot in those tunnels, if they exist.