Ah, the nostalgia. When I read this it made me dig out that probation notice from 24 years ago, and those pictures we took in the tunnels.
We were actually caught on our second trip down. Someone at a Physics Department picnic told us the way in: the control panel in the Welch elevator had been sufficiently loosened so that you could pull out a screw, swing out the panel, and trip the keyswitch to let you go into the basement. In the basement there was a chain-link gate, but the lower hinge had been rendered easy to remove, so you could (slightly) swivel the whole gate on the top hinge and the lock on the other side and crawl under.
We got in about five hours of exploring the first time. The second time we were dead-ended at a gate (trying to remove the hinges) when we were corraled by campus police (they actually yelled "freeze!"...scared the shit out of us). We dropped our tools behind the steam pipes and never recovered them.
We got probation, but really no other fallout.