There are two networks of tunnels on (or rather, beneath) Carleton's campus: utility service tunnels and pedestrian Tunnels. Both are referred to collectively as the tunnels.
Many of the buildings on the west side of the core campus are connected by utility service tunnels. Heating, electrical, and telecommunications conduits are located in these tunnels. These are not, and never have been, publicly accessible and are difficult to traverse for the workers who must use them. The routes of these tunnels can be traced in fall and spring by observing the sidewalks and grassy areas where snow melts.
Many buildings on the east side of Carleton's campus are linked by underground pedestrian tunnels. These tunnels connect the Old Music Hall, the Music and Drama Center, Nourse, Myers, Evans, Cowling, and Watson. (Gridley was also on this tunnel system.) The tunnels were originally intended to allow female students, housed on the east side of campus, to travel among their dorms after hours, and to provide them shelter from the weather. An entirely separate pedestrian tunnel joins Burton to Musser.
These tunnels also contain the heating, electrical and telecommunications conduits for the east side of campus.
The tunnels are best remembered for their art -- a patchwork of murals, signs, grafitti, and other painted illustrations on the walls, floors, and even ceilings of the tunnels. Tunnel art was a communal process accumulating over three decades, beginning in the 1960's and continuing through the 1980's. Peter Gunn '92 has taken photos of many of the paintings in the tunnels and has posted them at http://www.flickr.com/photos/pdbg/sets/329729/
For safety and insurance reasons, the tunnels were closed to the public, including students, in 1988, "until further notice", though some out-of-date college guidebooks still tout them as an attractive feature of Carleton's campus. There are no plans to reopen the pedestrian tunnels in the foreseeable future. Additional service conduits were installed after 1998, making them less friendly for pedestrian traffic.
A portion of the pedestrian tunnels are sometimes available for touring during Alumni Weekend.