Image via Bing Maps: binged.it/xrlH8T (Google Maps is out of date for this location)
Estimated server count: 204,160
Google secured the land for this data center in 2005 and it became operational in 2006. There are two buildings with an obvious space in the middle for a third. Permission has been granted for construction of the third building but it's not expected house compute servers. Instead it might be a tape storage center. Here's a great aerial photo of the installation. And some interesting background info about this site courtesy of Harper's Magazine.
The Google Maps imagery for this site shows nothing but empty space. Some people have suggested that this is deliberate concealment by Google. I think, however, it's more likely just an accident due to out-of-date imagery. If Google wanted to conceal their data centers surely they'd do so for all of them, not just one or two?
Return: All Google data centers
Google's own page about this data center: google.com/datacenters/the-dalles