The Pratt tunnel system (Brooklyn, NY) consists of two tunnels. One on the west side of the Main Building that connects to the Library and which is sealed with metal gates. The other is behind the mechanical room and connects with the Chem/Engineering Building in the basement. These tunnels are not especially noteworthy, however the latter tunnel contains the remnants of the pipe organ that used to be in memorial hall. The remaining flutes and pipes make great toys and noisemakers. It's beyond salvaging, so what the hey. The Pratt mechanical room is well worth a visit and is open to the public. It contains three working steam-powered electrical generators from the 1880s. They are beautiful pieces of machinery, with flywheels, governors, etc. In this vein, Pratt's chief engineer, Conrad Milster, hosts a New Years Eve whistle-thon with dozens of railroad, ship, and other steam powered whistles and horns.
A far superior tunnel system is at Wake Forest (Winston-Salem, NC). There is a huge network of large tunnels well laid out with little road signs of New York City streets put in by earlier intrepid tunnelers (pre
1977, when I was there). The best entry is from the basement of the public tunnel between the girls dorms. It is basically for steam delivery and communication, however, it also makes for great access to your girlfriend for a late rendez-vous.
Other good tunnels: There are a couple of tunnels at City College of New York, between Steinman Hall (the engineering building) and the Main Hall and then further to the south. You really have to look around to find the entrances however.
I also recommend the steam and other access tunnels north of Grand Central Station in New York. Here you have the quadruple danger of getting caught, run over by a train, stepping on the third rail, and the usual dangers of steam tunneling. Walk off the north end of the upper platforms, many of the passages are on the west.
A cautionary note on steam tunneling which was not explicit in the warnings I read on the site: Because the tunnels are generally enclosed
and usually have little cross ventilation, steam leaks can not only burn
you but can displace all the oxygen within a tunnel segment. You can't breath steam. I have walked into tunnels only to have to abandon them because it becomes impossible to continue without breathing. It may sound goofy, but I have to recommend the buddy system. A) to keep you from doing anything extremely stupid, B) you might need someone to save your life, cause if you pass out and you are alone, they won't find you for weeks (what's that smell?), C) if you get caught, they are less likely to through both of you out, and D) it is just more fun to be able
to laugh about it later with someone else in the know.
Good Luck and Happy Exploring
P.S. is there also any kind of site on "buildering", the exploration of building rooftops, carillons, bell towers, water towers, mechanical rooms, etc.? In this vein, I have to recommend the walk down from the 103rd floor to the 86th floor of the Empire State Bldg. There is a concrete core within the upper steel skin with catwalks over 15 stories of clear inter-space between the core and the skin. Great views of New York and building structure. Just duck under the chain after the elevator splits and the crowd disperses. Can't vouch for latest security though.
also..."fwiw, the city college of new york has a utility tunnel connecting 4 or 5 buildings on the "north campus" - nyc between 135 and 136 street and across a 3 block width. students used these tunnels in the 60s, and i assume that they are still available."