Official Google Blog: Traveling by zip-line
Well, to not spoil all the mystery, I suggest you head over to the Official Google Blog to read the post itself before you start here.
What? You've read it already? OK, good. My initial reaction to the post was "Umm ... if they were to build something of a permanent nature, ... will they get into trouble for that?"
As it turned out, they did end up having their construction project dismantled. However, it allows us a peek into what Googlers do and if they are representative of Google's working population, I don't mind meeting a few Googlers myself.