I went into the UT tunnels in the early 80's with a guy who lived in our dorm (Jester) - he had been down dozens of times, and liked to show off by taking groups of people down. I remember the access was a street level metal hatch near the center of campus, not far from the student union building. It was not locked, and it was surprisingly easy to spring open. The only prominent security then in the tunnels was near the Harry Ransom Center, where the big $$ exhibits are kept. I HOPE the security is much better now - and given the modern age, I would discourage anyone from criminally tresspassing today.