This is the place to share cool info on RPI tunnels, secret passage ways, etc.
For those of you who haven't explored much at all, there are tunnels between the J-ROWL, JEC, DCC and CII which are open to students 24/7. They can be a little tricky to find coming from the DCC end, but are where WRPI is.
But don't get deluded, there are not really any other tunnels on campus except for the tunnels connecting Sharp Hall and 2 other residence halls.
And the Steam tunnels::
As far as more adventurous explorations, So far, google searches have only returned this link for steam tunnels, which is not archived  Do note that being found in a steam tunnel (the other tunnels are fine) is grounds for immediate suspension/expulsion from the Institute, and is dangerous to begin with.
Added via anonymous account: I have been in the Ricketts steam tunnel. It was fun to see, but disappointing considering the effort and risk to get in it. There is a metal grate in front of Ricketts that, at least at the time (some time in 2006), was not secured. From it, there is a ladder down into the tunnel. In one direction, it leads to a doorway which opens into the Ricketts basement, and another locked door to the 87' gym. The other direction appears to go towards the administration building and Walker lab, but the tunnel became too narrow to continue in that direction very far.
If you're up for an adventure, it's fun to know you're one of the few who has been down there, but if you've done even the slightest bit of tunnel hacking, you'll be disappointed.
Added via another anonymous account: on a public safety ride-along in 2008, a public safety officer explained that there are several other tunnels that students dont have access to. During an explanation of the readiness procedures for EMPAC, The officer stated that (direct quote) "It is possible to get from public safety to EMPAC without ever seeing the light of day". These statements have not been confirmed.