Butler tunnel system
The Butler tunnel system is a series of tunnels connecting Butler Library, Carman Hall, Lerner Hall, and John Jay Hall. Presently, only the tunnels below Butler, Carman, and Lerner can be accessed. The door to John Jay is locked. Some have speculated this tunnel system also connects into Furnald Hall and Hogan Hall.
General discussion about the tunnel system
This tunnel system dates to the years when Ferris Booth Hall was still standing, and has circuit breakers labelled "FBH" to prove it. As a result, the tunnels features some of the 'awesomest' graffiti in the entire Columbia tunnel system. There is also a weird room under Carman Hall with a bunch of old, discarded computers and printers. You can feel free to score a Little Tykes car which is missing two wheels.
If you manage to get in here during a time when the bookstore's loading gate (metal grate all the way at the west end of the tunnel, straight down from the Butler entrance to the tunnel) is open, you can also access the pump room that serves Lerner and a storage area behind the bookstore. What is believed to be staircase Y from Carman ends down here. But even if the gate is closed, you can still have fun poking through a number of boxes in here that are filled with the bookstore's paperwork and some broken-down display racks and things. There is also a door next to this metal gate which leads into a break room with lockers and a couch, and it's possible to get into the bookstore from this room.
If you feel like you got stuck in here, don't fret. There is a sure-fire way out of this tunnel. Follow the tunnel up the incline plane in the direction of the pipes that are marked "Broadway/Hogan." That dark area at the end of the elevation does not go farther than 10 feet, and all there is there is trash (unused lockers). There is a door to get out to your left side as you approach the darkness. This door will dump you outside in front of Furnald, although you can't reenter this way because there is no doorknob on the outside.
From Carman lobby, take the main staircase down two levels (one level below laundry). When you reach the bottom of the staircase, the (locked) door to the tunnels will be on your right. There will also be an unlocked door to the carman elevator shaft. The other (locked) roll-up gate leads to the cargo room, which is also cool to visit. This room can sometimes be accessed by taking the service elevator down to the bottom floor (floor C). All doors leading out of Carman are locked, unless you get into the loading dock.
From Butler lobby, take a right past the Lounge. Take the stairs around the corner down till you get to the steel gate. If the gate is closed, you may need a screwdriver to pop it open (very crappy lock - it's not even a deadbolt). When you get to the tunnel level of Butler, head to your right as far as you can go. You'll end up in front of a set of double doors (which used to be padlocked shut, but are unlocked as of this writing) leading to Carman, Lerner, and that creepy staircase which dumps you in front of Furnald. At the other end of the hallway is a door which is permanent marked "John Jay." It is both locked and hot-glued shut, but if you're there in the evening when either JJ's Place or the dining hall is open, you may be able to hear voices from people working down there.