This tunnel is in a high traffic area; if you choose to explore it, be very careful.
If you enter at Hillcrest at the manhole in front of Hillcrest, you will find yourself in a small, cramped tunnel. Not to worry though, it widens up as you continue down it. You will eventually find yourself at a turn into the main tunnel, which runs beneath the sidewalk along West Campus Drive. You may notice steam pipes running into the wall in the opposite direction of the tunnel; it appears that these turn toward the Fralin-Dietrick tunnel, probably connecting just where that tunnel becomes inaccessible near Fralin.
If you continue down West Campus Drive, you will pass the exit to the grate in front of Litton Reaves on your right.
Eventually, you will find yourself at another turn. At this point, you are beneath the roundabout at the end of West Campus Drive. The main tunnel continues west beneath the sidewalk along Washington Street and a subtunnel continues straight, running next to McComas.
Right after the subtunnel, which you will have to climb over pipes if you choose to explore, you may notice a device that discharges steam. Don't hang around here, but if you move quickly, you should be okay.
The tunnel continues straight until you reach a dead end beneath a manhole. This manhole is across the street from the Greenhouses. From here the pipes diverge; one set runs off underneath Washington Street and the other set runs straight, probably connecting to the Life Sciences-Bioinformatics run.
From here, I recommend you turn around and exit either at the Litton Reaves grate or back at the Hillcrest grate. Unfortunately, this tunnel does not have many good entrances or exits. As usual, be very careful to make sure no one is around when you exit the tunnel.