I've lived in C-U for more than 20 years, and attended the U of I, and it wasn't until recently that I was even aware that these steam tunnels exist, and are apparently a rather common way to heat buildings on university campuses.
The tunnels are sort of an urban legend, mostly because those that are still active are off limits to everyone except authorized Facilties and Services employees, you know, because of danger. This map, with the tunnels drawn in and notations designating easy access points was posted on Reddit and dubbed the UIUC Marauder's Map.
Image from Reddit.
The Daily Illini published an informative article about the tunnels a few years ago:
This system, based on nine miles of steam tunnels, serves as the University’s prime energy source. The tunnels also allow the University to more easily connect newly constructed buildings to the system. The system of tunnels isolate the steaming hot pipes so that Facilities and Services can maintain, detect and fix possible steam leaks.
“Abbott Power Plant burns coal, natural gases and water, which we then burn to make steam, rather than condense it,” said Daniel Ruzic, professor of nuclear, plasma and radiological engineering at the University. “The steam then travels through the steam tunnel system to provide energy to the buildings connected.”
Here's a mildly entertaining article from students who found a way into the tunnels (with blurred faces and pseudonyms).
And here's a photo of one of the tunnels under construction in 1934.
Photo from UIUC Archives.
Editor's Note: We do not condone unlawfully entering the steam tunnels.
Top image from Reddit.