Columbia has an extensive tunnel system connecting most buildings on campus and acting as conduits for steam, electricity, telecommunications, and other infrastructure. It is a foreboding place where unspeakable acts of pure horror are committed.
The oldest tunnels are from the mental asylum that existed before the Morningside Heights campus was built. These tunnels are small and extremely hot, and they connect to Buell/La Maison Francaise, the one building remaining from the asylum. The steam tunnel system between Hamilton, Kent, Philosophy, and Fayerweather connects to these old tunnels.
Mudd/Uris/Havemeyer tunnel system
Another steam tunnel system connects Mudd, Uris, Dodge Fitness Center, and Havemeyer, and is generally considered the easiest to access. These tunnels contain old rail track that was used to transport coal for heating. They also contain the "Signature Room," where one can find many quotes and names left by previous tunnelers. As with all tunnels on campus, these contain many more secrets for students to discover. This tunnel system also used to connect to the first floor of Pupin Hall, but that way was blocked in the 1990s.
Until the Summer of 2003, the first floor of Pupin was virtually untouched from the last days of the Manhattan Project. Notes and daily logs scattered dusty tables. Half-completed experiments sat in stasis, only visible to the few explorers who got in. Since 2003, the first floor has been cleaned out, and is now mostly empty. However, one of the first cyclotrons remains, though it is difficult to access.
Other tunnel systems
Another tunnel system connects Havemeyer, Math, Lewisohn, and the Miller Theater.
Tunnels also connect Butler, Carman, Lerner, and supposedly Furnald and Hogan. John Jay/Hartley/Wallach can also be reached from this system, but the door is locked, and the tunnels connect to JJ's Place, making exploration difficult.
A tunnel between Butler and Low is rumored to exist, but no evidence of such a tunnel has ever been found. Any entrance to this tunnel from Butler is well hidden or heavily locked, and the Low entrance would be next-to-impossible to find given that Campus Security is based on the first floor.