A couple of my friends and I have come up with a theory.
It all started when we were thinking of good places to hold meetings; we all thought that it's not a meeting unless it's behind a door with a little sliding panel and a surly guard to ask, "Password?"
It grew from that into being a secret underground bunker.
We decided, since the University was built during the Cold War era, that there must be a secret underground bunker hidden somewhere on campus. We concluded that it must have been for the Vice–Chancellor and is located under Senate House, since this is where his office was before it moved, presumably, to University House.
I've long since suspected that the Twisty–Metal–Tube–Thing in the grass surrounded by Senate House and the Chaplaincy was a disguised air–vent for a secret underground bunker, and this fits in rather well with this new theory.
I've looked on the Central Timetabling website and I can't see it, so I'm not sure how we'd book it; I'm sure if we asked anyone they'd swear it didn't exist…