"MIT Tunnel Tour: A subterranean landscape of the campus
5:30 pm and 9 pm (45 minutes)
One-sixth of a mile long, the Infinite Corridor is the backbone of a subterranean system connecting labs, classrooms, offices, and libraries. Legend hails it as the third longest tunnel system in the world, after the corridors of the Kremlin and the Pentagon. Come explore guided by the MIT Student Ambassadors. "
That almost takes some of the fun out of it.
ETA: Clarification from someone more knowledgable than me brought to light the fact that MIT has a public/open tunnel system -- described above -- and a steam tunnel system which is not public. So, UIUC doesn't have the equivilant of the 'open' tunnel system, and the steam tunnel system maps more closely to the only existing UIUC tunnels. I just didn't realize they were seperate.