There are different levels of secrets at Google. Almost everything unreleased is “confidential” - which means that we don't talk about it to the outside world. Then there is the “top secret” stuff - stuff that you don't even talk about to other Googlers.
Now, top secret stuff is rare because it's a little poisonous. An environment where lots of things are secret between coworkers isn't a pleasant one. How we cool our data centers was one of those items and I was sworn to secrecy when I was lucky enough to be given a guided tour of our Oregon operations.
But, for whatever reasons, this information is now public! Seriously, this is some of the coolest (no pun intended) stuff that Google does: go read about evaporative cooling.