The clock.

Supposedly it's under the City, though most people find it difficult to get to the centre of it all. There are monsters in the Underground and it's sort of a maze down there, so the City's defending it quite a bit. As for what it does or means, Merlin knows. I've heard people say the ticking's a countdown to the end of all the worlds, but... There've been times when the ticking's stopped entirely and you lot still turned up from home, so maybe it isn't that.